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Piston Ringsfor Combustion Engines

S E R V I C E

T I P S

&

I N F O S


2/802 | Piston Rings

2nd Edition 01.2010Article Number 50 003 958-02ISBN 978-3-86522-492-7

Edited by:Motor Service Technical Market Support

Layout and production:Motor Service MarketingDIE NECKARPRINZEN GmbH, Heilbronn

This document must not be reprinted, duplicatedor translated in full or in part without our priorwritten consent and without reference to thesource of the material.

All content including pictures and diagrams issubject to alteration. No liability accepted.

Published by: MS Motor Service International GmbH

I BRING YOU THEPOWER OF KOLBENSCHMIDT,PIERBURG AND TRW ENGINE

COMPONENTS!

Motor Service Group.Quality and Service from a single source.The Motor Service Group is the sales organisation for the worldwide aftermarketactivities of Kolbenschmidt Pierburg. It is one of the leading suppliers of enginecomponents for the independent aftermarket including the premium brandsKOLBENSCHMIDT, PIERBURG and TRW Engine Components. Our comprehensiveproduct range allows our customers to procure engine components from a singlesource. As a problem solver for dealers and garages, Motor Service o ers extensiveservices and the technical expertise that you would expect f rom the subsidiary ofone of the largest automotive suppliers.

Kolbenschmidt Pierburg. Renowned supplier to theinternational automotive industry.As long-standing partners to the automotive industry, the companies in theKolbenschmidt Pierburg Group develop innovative components and systemsolutions with acknowledged competence in air supply and emission control,foroil and water pumps, for pistons, engine locks and engine bearings. The productscomply with the high demands and quality standards of the automotive industry.Low emissions, reduced fuel consumption, reliability, quality and safety theseare the forces that drive innovation at Kolbenschmidt Pierburg.

LiabilityAll information in this brochure has been carefully researched and compiled.Nevertheless, it is possible that er rors have occurred, information has beentranslated incorrectly, information is missing or the details provided havechanged in the intervening time. As a result, we are unable to provide anyguarantee nor to accept any legal liability for the accuracy, completeness,currency or quality of the information provided. We hereby waive all liabilityfor any damages, whether direct or indirect in nature and whether tangible orintangible, resulting from the use or misuse of information or from incompleteor incorrect information in this brochure, unless proven to be the result ofdeliberate intent or negligence on our part.

Likewise, we shall not be liable for damage arising because the enginereconditioner or mechanic does not have the necessary technical expertise,the required knowledge of, or exper ience in repairs.

The extent to which the technical methods and repair information describedhere will apply to future engine generations cannot be predicted and must beveri ed in individual cases by the engineer servicing an engine or the workshopoperator.



Pictograms and symbolsThe following pictograms and symbols

are used in this brochure:

Reference to useful advice, explanations and

supplementary information for handling.

Caution Draws your attention to dangerous

situations with possible personal injuries or

damage to vehicle components.

Preface

The issuePiston rings have been in use for as long as combustion engines themselves. Despitethis, ignorance or inadequate knowledge of piston rings is still frequently evident today.No other component is so critical when power loss and oil consumption are at stake. Withno other component in the engine is the divide between expectations and utilised capitalgreater than when replacing piston rings.

All too often, con dence in piston rings su ers due to the exaggerated demands madeon them. Consequently - and even against ones better judgement - halftruths and lies,clichs and false estimations remain in force in repair workshops and among consumers.

However, piston rings are most often adversely a ected by cheap repairs (for instanceby reusing worn-out interacting sliding parts) and by unquali ed installation.

This brochureIn this brochure we have taken a user approach to the topic of piston rings. We havedeliberately refrained from concentrating too deeply on structural data and have insteadfocused on the more practical aspects. Wherever we do address design-engineering anddevelopment topics, this is simply for better understanding or supplementary to otherinformation.

The content of this brochure deals mainly with piston rings for personal and utilityvehicles. Engines that were originally designed for automobile applications, but havebeen installed e.g. in ships, railroad engines, construction machines and stationarymotors, are of course also included in this category. Besides a section with basictechnical principles, the practical section Installation and Service contains detailedinformation on tting and replacing piston rings as well as useful related topics suchas lubrication, oil consumption and running in the engine.

Successful repair and maintenance work is not di cult if you are aware of theinterrelations inside the engine. We point out what is necessary to assure that repairsare a success, but also show what can happen if certain rules are not observed.



Basics | 1

1.1Requirements onpiston ringsPiston rings for combustion enginesmust meet all requirements ondynamic linear sealing. They must notonly withstand thermal and chemicalin uences, but also achieve a seriesof functions and characteristics which

are listed below:

Functions: Preventing (sealing) the passage of gas

from the combustion chamber into thecrank case to avoid loss of gas pressureand, consequently, of engine perfor-mance.

Sealing, i.e. preventing the passage oflubricating oil from the crankcase intothe combustion chamber.

Ensuring an exactly defined thicknessof lubricating film on the cylinder wall.

Distributing the lubricating oil over thecylinder wall.

Stabilising the piston movements(piston rocking), in particular wheneverthe engine is cold and the runningclearance of the pistons in the cylinderbore is still great.

Heat transfer (heat dissipation) fromthe piston to the cylinder bore.

Characteristics: Low frictional resistance to prevent

excessive loss of engine performance. Good stability and wear resistance

against thermo-mechanical fatigue,chemical attack and hot corrosion.

The piston ring should not cause anyexcessive wear to the cylinder bore,because the engines service life hasbeen greatly reduced.

Long service life, operational safety

and cost effectiveness throughoutthe entire operating time.

In addition, piston rings have furtheressential tasks which are describedfrom of Chapter 1.6. Functions andCharacteristics.



Fig. 1

1.2 | The three main functions of piston rings

1.2.1Sealing of combustiongasesThe main task of compression rings is toprevent the passage of combustion gasbetween piston and cylinder wall into thecrankcase. For the majority of engines,this objective is achieved by two com-pression rings which together form

a gas labyrinth.

For design reasons, the tightness of pistonring sealing systems in combustionengines is below 100%; as a result a smallamount of blow-by gases will always pass

1.2.2Scraping and distributing oilNext to sealing the area between thecrankcase and combustion chamber, thepiston rings are also used to control the

oil lm. The oil is uniformly distributedonto the cylinder wall by the rings. Mostexcess oil is removed by the oil controlring (3rd ring), although the combinedscraper-compression rings (2nd ring)also remove the oil.

Fig. 2

by the piston rings into the crankcase. Thisis however a normal state which cannot becompletely avoided due to the design. It isessential though, to prevent any excessivetransfer of hot combustion gases past thepistons and cylinder wall. Otherwise thiswould lead to power loss, an increase ofheat in the components as well as a loss oflubricating e ects. The service life and thefunction of the engine would consequentlybe impaired. The various ring and sealing

functions as well as the produced blow-bygas emission are addressed in thefollowing chapters.



1.2.3Heat dissipationTemperature management for the pistonis another essential task of the pistonrings. The major portion of the heatabsorbed by the piston during thecombustion process is dissipated by thepiston rings to the cylinder surface. Thecompression rings, in particular, aresigni cantly involved in heat dissipation.50% of the combustion heat absorbed bythe pistons is already dissipated to thecylinder wall by the upper compressionring (depending on the engine type).

Without this continuous heat dissipationby the piston rings, a piston seizure in thecylinder bore would occur within a fewminutes or the piston even melt. From thisperspective, it is evident that the pistonrings must always have proper contactto the cylinder wall. Whenever out-of-roundness is caused in the cylinder boreor if the piston rings are jammed in thering groove (carbonfouling, dirt,deformation), it will only be a matter of

time until the piston su ers fromoverheating due to a lack of heatdissipation.

The three main functions of piston rings | 1.2

55 %

30 %

15 %

Fig. 3



1.3 | Types of piston rings

1.3.1Compression rings

Rectangular ringsThe term rectangular ring relates to ringswith a rectangular cross section. Both ringsides are parallel to each other. This ringtype is the simplest kind of compressionring and the most widespread.Nowadays, it is predominantly used as

rst compression ring on all petrol enginesfor passenger cars and sometimes also ondiesel engines for passenger cars. Insidebevels and inside steps cause the ring to

twist in its installed (tightened) condition.The position of the bevel or the inside stepon the upper edge produces a positivering twisting. Details on the exact e ectsof this twisting are given in, see chapter1.6.9 Ring twisting.Rectangular ring

Rectangular ring with inside bevel

Rectangular ring with inside step

Compression rings with oilscraping function

These rings have a double function.They support the compression ring whensealing o the gas and the oil control ringwhen regulating the oil lm.

Taper faced ring

Fig. 1

Important note:Taper faced rings are used on all type ofengines (passenger cars, utility vehicles,petrol and diesel engines), mostly in the

second ring groove.



The contact surface of taper faced ringsis conical. The angular deviation relatedto the rectangular ring is between approx.45 and 60 angular minutes, depending onthe version. Due to its shape, the ring in itsnew condition only supports at the loweredge and as a result, is only in contactwith the cylinder bore at a certain point.This brings about a high mechanicalsurface pressure in this area and a desiredremoval of material. Even after a short

runtime, this intentional running-in wearresults in a perfect round shape and,consequently, in an excellent sealinge ect. After a prolonged runtime of some100,000 miles, the conical contact surfaceis removed as a result of wear, so thatthe taper faced ring rather performs thefunction of a rectangular ring. The ringformerly produced as a taper faced ringnow does a good sealing job as arectangular ring.

The fact that the gas pressure also acts onthe ring from the front (the gas pressurecan enter the sealing gap between cylinderwall and piston ring contact surface)slightly reduces the gas pressurereinforcement so that a slightly reducedcontact pressure is applied during therun-in time of the ring and running-in issmoother with less wear (Fig. 2).

Taper faced rings also have good oil

scraping properties in addition to theirfunction as a compression ring. This is aresult of the recessed upper ring edge.During the upstroke from the bottom to thetop dead centre, the ring slides on the oil

lm, and thus oats a bit on the cylindersurface due to the hydrodynamic forces(formation of lubricating wedge). Duringthe movement in the opposite direction,the edge penetrates deeper into the oil

lm and scrapes o the oil, mainly intothe direction of the crankcase. Taper faced

rings for petrol engines are applicatedinside the rst ring groove.

Types of piston rings | 1.3

The position of the bevel or inside stepon the lower edge causes a negative ringtwist in this case (see chapter 1.6.9 Ringtwisting).

Fig. 2

Taper faced ring with inside bevel at thelower edge

Taper faced ring with inside step at thelower edge



Napier rings On the napier ring the lower edge of thepiston ring sliding surface has a rectangularor an undercut recess, which not onlyseals o the gas, but also acts as an oilscraper. The recess forms a certainvolume in which the scraped-o oil cangather prior to returning to the oil pan.In the past, napier rings were used as

second compression rings on manyvehicle engines. At present, taper facednapier rings are mainly used instead ofnapier rings. Napier rings are also used incompressor pistons for air brake systems,in particular as a rst compression ring.

Napier ring

Taper faced napier ring

The taper faced napier ring is anadvancement of the napier ring.The oil scraping e ect is enhancedby the tapered sliding surface.

On piston compressors, the taper facednapier ring is not only tted in the secondbut also in the rst ring groove.

The circumferential nose stops before thejoint end, thus improving the gas sealingfunction. Compared to the normal taper

faced napier ring, this reduces the blow-bygas emission (see also 1.6.5. Closed gapclearance).Taper faced napier ring with closed joint




Keystone ringsKeystone rings or half keystone rings areused to prevent any carbon deposits in thering grooves and therefore to counteractany seizing-up of the rings in the ringgrooves. In particular when extremelyhigh temperatures are produced eveninside the ring groove, there is the hazardthat the engine oil in the ring groove willcarbonise due to the temperature exposure.On diesel engines, soot may be generated

in addition to a potential oil coking, whichalso encourages deposits

Cleaning functionDue to the shape of the keystone rings

and their movement in the ring grooveas a result of piston rocking (see chapter1.6.11. Piston ring movements) carbondeposits are mechanically ground away.

On keystone rings, the two anks of thering are not parallel but are trapezoidalto each other. The angle is normally 6 ,15 or 20 .

On half keystone rings, the lower ankhas no angle and is at right angles tothe piston ring sliding surface.

Full keystone ring

Half keystone ring

in the ring groove. If the piston ringsseize-up as a consequence of depositsin the groove, the hot combustion gasescan ow unimpeded between the pistonand cylinder wall, thereby heating themup.Piston head melting and severe pistondamage would result. Due to highertemperatures and soot formation on dieselengines, the keystone ring is preferablyused in the rst ring groove, but

sometimes also in the second groove.

AttentionKeystone rings (half or full) should notbe used in normal rectangular grooves.Whenever keystone rings are used, thering grooves to be tted on the pistonmust always have the appropriate shape.

Fig. 1 Fig. 2




Function:Oil control rings are designed for the solepurpose of distributing oil on the cylinderwall and scraping excessive oil o thecylinder wall. Oil control rings usuallyhave two scraping lands for improvingtheir sealing and scraping functions. Eachof these lands scrapes excessive oil o thecylinder wall. A certain volume of oil isproduced both on the lower edge of the oilcontrol ring and between the lands, which

must be removed from the vicinity of thering. As far as the rocking movement ofthe piston inside the cylinder bore isconcerned, the smaller the distancebetween the two ring lands, the betterthe sealing function.

It is essential that the oil volume, whichis scraped o the upper scraping landand accumulates between the lands, isremoved from this area, as otherwise itmay reach the top of the oil control ring

where it must be scraped o by the secondcompression ring. For this purpose, one-and two-piece oil control rings are eitherprovided with longitudinal slots or boresbetween the ring lands. The oil scrapedo the upper land is directed throughthese holes in the ring body to the rearof the ring.

The scraped oil can be drained from thereby di erent ways. One method is to directthe oil through the bore in the oil scrapinggroove to inside the piston so that it candrip back into the oil pan from there(Fig. 1). With so-called cover-slots (Fig. 2and 3), the scraped oil is returned throughthe recess around the piston pin boss onthe outer side of the piston. However, acombination of both versions is also used.

Both versions are well-proven for drainingthe scraped oil. Depending on the pistonshape, combustion process or the intendedapplication, either version can be used.A general statement cannot be made infavour of one or the other version on amere theoretical basis. The decision asto which method is better suited for therespective piston is therefore taken onthe basis of practical test runs.

Fig. 2

Fig. 3

Fig. 1

1.3.2Oil control rings


Important note:On two-stroke engines, the lubrication ofthe piston is e ected using a lubricationmixture. Due to the design, there is noneed for an oil control ring to be used.



Ring Types:One-piece oil control ringsOne-piece oil control rings are no longerused in modern engine manufacture.One-piece oil rings obtain their tensionsolely from the piston ring cross section.These rings are thus relatively rigid andhave a less favourable form matchingcapability and, as a result, a suboptimalsealing behaviour than multi-piece oilcontrol rings. One-piece slotted oil control

rings are of grey cast iron.

Slotted oil control ring Basic version with rectangular scrapinglands and slots for draining oil.

Double-bevelled oil control ring Compared to the slotted oil controlring, both edges of the sliding landsare chamfered to achieve an improvedsurface pressure.

Top bevelled oil control ring The lands of this ring are only chamferedon the combustion chamber side. Thisresults in a stronger oil scraping e ectduring the downstroke of the piston.

Fig. 4




Two-piece oil control rings comprise aring body and a spiral spring arrangedbehind. The ring body has a signi cantlysmaller cross section compared to theone-piece oil control ring. As a result, the

ring body is relatively exible and has anexcellent form matching capability. Thebed of the spiral expander on the insideof the ring body is either semi-circular orV-shaped.

The actual tension is provided by a coilspring made out of heat-resistant springsteel which rests behind the ring andpresses it against the cylinder wall.In operation, the springs adhere closelyto the rear end of the ring body, forming

a unit. Although the spring is not twistedagainst the ring, the entire ring unit turns

freely in the piston ring groove - thesame as other rings. The radial pressuredistribution with two-piece oil controlrings is always symmetrical, becausethe contact pressure along the entire

perimeter of the spiral spring is alwaysidentical (for more information see alsoChapter 1.6.2 Radial pressure distribution).

To increase the service life, the outerdiameter of the springs are ground, coiledtighter at the ring joint or even coated witha Te on hose. Thanks to these measures,the frictional wear between ring body andspiral spring is reduced. The ring bodiesof two-piece rings are made of either greycast iron or steel.

Two-piece oil control rings (spiral expander type)

Fig. 1


Important note:The free gap this is the distancebetween the joint ends of the ring bodyin its dismantled state without the expander

spring arranged behind - is insigni canton multi-piece oil control rings. In part icularon steel rings, the free gap can be practicallyzero. This is however neither a de ciencynor a reason for complaint.



Slotted oil control ring with spiralexpander Basic version with better sealing e ectthan on a one-piece slotted oil controlring.

Spiral expander top-bevelled oilcontrol ring

Same shape of sliding surface as a top-bevelled oil control ring, however withimproved sealing e ect.

Double-bevelled spiral expander ring Same shape of sliding surface as a double-bevelled oil control ring, however withimproved sealing e ect. This is the mostcommon oil control ring and can be used

in any type of engine.

Double-bevelled spiral expander ring with chromium-plated landsSame characteristics as for the double-bevelled spiral expander ring, butwith increased wear resistance, andconsequently, a longer service life. Itis especially suited for diesel engines.

Double-bevelled spiral expander ring made of nitride steelThis ring is made of a wound strip of apro led sectional steel and is providedwith an antiwear protective coat on allsides. It is extremely exible and lesssusceptible to fracture than the cast ironrings mentioned above. The oil drainagebetween the rails is made through stampedround ori ces. This type of oil control ringis mainly used on diesel engines.




Three-piece oil control ringsThree-piece oil control rings are made oftwo lamellas which are pressed againstthe cylinder wall by a spacer and expanderspring. Steel rail oil control rings areavailable either with chromium-platedsliding surfaces or nitrided on all sides.The latter contribute to improving thewearing properties of the sliding surfaceas well as between and the rails(secondary wear). Three-piece oil control

rings have an excellent form matchingcapability and are mainly used on petrolengines for passenger cars.

Various expander spring versions

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Installation condition




Compression ring

Fig. 6

Compressionand scraper ring

Oil control ring

Friction Running-in Service life

High ring tension

Low ring tension

Wear-resistant material

Softer material

Flat ring heights

Large ring heights

favourable - positive medium - neutral unfavourable - negative

1.3.3Typical piston ringassemblyThe complex requirements made on piston rings cannot be met by one piston ringalone. This can only be achieved bycombining several piston rings of variousdesigns. In present-day vehicle enginemanufacture, a combination of compression

ring, combined compression and scraperring and a simple oil control ring hasprooved the best results in practice (Fig. 6).Nowadays, pistons with more than threerings are relatively rare. Using more thantwo compression rings does not improvethe sealing quality, but rather increasesthe friction losses.

1.3.4The best piston ringThere is neither the best piston ring,nor the best piston ring assembly. Eachpiston ring design is a specialist in its

eld. Each ring type and ring compositionis ultimately a compromise betweencompletely di erent, and sometimes

opposing, requirements. Changing onlyone of the piston rings may unbalancethe entire ring set matching. The nalpiston ring matching for an engine to benewly designed is always de ned on thebasis of extensive test runs conductedboth on the test rig as well s under normaloperating conditions. The following chartbelow does not claim to be complete,though it does show the e ects ofdi erent ring characteristics on thevarious ring functions.




Free gap

Joint ends

Piston ring internalsurface

Ring ank surface

Piston ring sliding surface

Ring back(opposite to the joint) ends)

Closed gap (cold clearance)

Cylinder bore diameter

Radial wallthickness

Radial clearance

Groove height

Axial clearance

Piston ring height

Groove base diameter

Cylinder bore diameter

1.4 | Piston ring designations



1.5.1Piston ring materialsThe materials for piston rings are chosenon the basis of running properties andconditions the piston rings are subjectedto in operation. Good elasticity andcorrosion resistance are as essential ashigh resistance against damage underextreme operating conditions. Even today,grey cast iron is still the main material formaking piston rings. From a tribologicalviewpoint grey cast iron and the graphiteinclusions in the structure presentexcellent dry-running properties (drylubrication by graphite). These are ofparticular importance whenever thelubrication by engine oil is no longerwarranted or the lubricating lm is alreadydestroyed. In addition, the graphite veinsinside the ring structure function as anoil reservoir and thus counteract thedestruction of the lubricating lm underadverse operating conditions.

The following materials are used as greycast iron materials: Cast iron with lamellar graphite

structure (lamellar graphite cast iron),tempered and untempered.

Cast iron with globular graphitestructure (nodular cast iron), temperedand untempered.

Piston ring casting process

Chromium steel with martensitic microstructure and spring steel are used assteel materials. The surfaces are hardenedto increase wear resistance. Hardeningis generally done by nitriding*.

Structure and shape of piston rings | 1.5

* Nitrogen hardening (nitriding) in technical jargon is also called nitrogenising (addition of nitrogen) and is a steel hardening process. Nitriding is normally carried out at temperatures betweenapprox. 500 and 520 C with treatment times from 1 to 100 hours. Diffusing nitrogen causes the formation of an extremely hard compound layer of iron nitride on the workpiece surface. Dependingon the treatment time, this layer can be 1030 m thick. Common processes are salt-bath nitriding (e.g. crankshafts), gas nitriding (for piston rings) and plasma nitriding.



1.5.2Surface coating materialsThe sliding lands or sliding surfaces ofpiston rings can be coated to improvetheir tribological* characteristics. In thisprocess, the main priority is to increasethe wear resistance as well as to assurelubrication and sealing under extremeconditions. The coating material mustharmonise both with the material of thepiston ring and cylinder wall and thelubricant.

Properties: High temperature resistance Good dry-running properties Softer than chromium Less wear-resistant than chromium rings

(more sensitive to dirt) More sensitive to ring flutter (molybde -

num separation may be caused by

extreme strain, such as for instance inthe event of knocking combustion andother abnormal combustion conditions).

Molybdenum coatingsTo protect the rings against scu ng,the sliding surfaces of compression rings(not of oil control rings) can be lled orcoated all over with molybdenum. Flamespraying or plasma spraying processescan be used for this purpose. Thanks to itshigh melting point (2620 C), molybdenum

warrants a very high temperature resistance.A porous material structure is also achievedas a result of the coating process. Engineoil can accumulate in the micro-cavitieson the sliding surfaces of the rings (Fig. 2)ensuring that engine oil is still availablefor lubricating the piston ring slidingsurface even under extreme operatingconditions.

Fig. 1

Fig. 2

Surface coatings on piston rings arewidely used. The rings of series enginesare frequently provided with chromium,molybdenum and ferro-oxide.

But rings with CCC (chromium ceramiccoating) and using the PVD process(physical vapour deposition process)are also used. Titan nitride (TiN) andchromium nitride (CrN) are applied insmaller production series (particularly

in racing engines).

1.5 | Structure and shape of piston rings

* Tribology (Greek: friction law) includes the field of research and the technology of interactive surface in relative movement.It deals with the scientific definition of friction, wear and lubrication.



Properties: Long service life (wear-resistant). Hard insensitive surface. Reduced surface (approx. 50 %

compared to uncoated rings). Good resistance to burn marks. Reduced dry-running properties

compared to molybdenum coatings. Thanks to its good wear resistance,

the run-in times are longer than withnon-reinforced rings, steel rail oil

control rings or U-Flex oil control rings.

Chromium plating Chromium platings can be applied ingalvanic processes as well as by plasmaspraying. Galvanic coating is used for oilcontrol rings.

Fig. 3


1.5.3Coating versions on pistonring sliding surfaces

fully coated chambered chambered on one sideFig. 5 Fig. 6Fig. 4



Fig. 1

Fig. 2

Fig. 3

1.5.4Separation of coatingsCases of separated surface coatingsappear from time to time on sprayedmolybdenum and ferro-oxide coatings.This is mainly due to faults when installingthe pistons (excessive expansion during

tment on the piston and tment of therings, as shown in Fig. 1). If the rings areincorrectly tted on the piston, the coatingwill only break out of the back of the ring(Fig. 2). If the coating akes o at the jointends (Fig. 3), this indicates ring utter dueto abnormal combustion (e.g. knockingcombustion).




Fig. 4

Fig. 5

Fig. 6

1.5.5Machining of slidingsurfaces (lathe-cut,lapped, ground)Cast iron piston rings are normally only

ne-turned on their sliding surfaces. Asa result of the short run-in time for non-reinforced rings, grinding or lapping thesliding surface is not required. If the

surfaces are coated or hardened, thesliding surfaces are either only groundor also lapped. The reason for this is that,because of the high wear resistance, itwould take a long time until the ringsadopted a round shape and sealed properly.This would result in power loss and highoil consumption.

1.5.6Ball-shaped slidingsurface shapesAnother reason for using grinding orlapping processes is the shape of thesliding surface. Due to up- and downstrokeand the movement of the ring in thegroove (ring twisting), rectangular piston

rings adopt a ball-shape on their slidingsurfaces after a while (Fig. 5 und 6). Thishas a positive e ect on the formation ofthe lubricating lm and the service life ofthe rings.




Symmetric, ball-shaped piston ringsliding surfaces (Fig. 1), irrespective ofwhether caused by running in the engineor already during the ring production, haveexcellent sliding properties and generatea lubricating lm of de ned thickness.With symmetric convexity, the thicknessof the lubricating lm is uniform duringthe up- and downstroke of the piston.The forces acting on the ring and makingthe ring oat on the oil lm are the same

in either direction.

If the convexity has already been createdin the course of ring production, it ispossible to create an asymmetric convexityto provide an improved control of oilconsumption. The apex of the convexitywill then not be in the middle of the slidingsurface but slightly below it (Fig. 2).

During the production process of coatedrings, they are already given a slightlyconvex or ball-shaped. As a result, theydo not have to gain their required shapeby running-in, but have their speci edshape and a practically run-in slidingsurface from the outset. Consequentlythe high rate of run-in wear as well as theassociated oil consumption is eliminated.Due to the contact of the piston ringsliding surface a certain point, a higher

speci c contact pressure on the cylinderwall is achieved and thus an improved gasand oil sealing e ect. The risk of edgeriding due to ring edges still sharp is alsoreduced. At any rate, chromium ringsalways have bevelled edges to prevent theoil lm from pressing through during therun-in time. In some unfavourable cases,the extremely hard chromium layer couldcause considerable wear and damage tothe much softer cylinder wall.

Fig. 1 - Symmetric convexity

Fig. 2 - Asymmetric convexity




1.5.7Surface treatmentsDepending on the design, the surfacesof piston rings can either be uncoated,phosphated or copperplated. This onlya ects the corrosion behaviour of therings. Although uncoated rings are niceand shiny in their new condition, theyare completely unprotected against rustformation. Phosphated rings have a dull

black surface and are protected againstrust formation by their phosphate coat.

The ring then slides properly on the oillm during the upstroke in the direction of

the top dead centre, because as a result ofthe larger e ective surface above the apexof the ring, the formation of the lubricatingwedge is, larger than below it (Fig. 3). Thering is pressed away from the oil lm thanmore than vice versa. This means that thethickness of the lubricating lm is notminimised as much during the upstroke.During the downstroke of the ring (Fig. 4),

the ring cannot oat on the oil lm to thesame extent because of the smallere ective surface below the apex. A largerquantity oil is scraped and returned to thecrankcase. Asymmetrically convex shapedrings are therefore intended for controllingthe oil consumption on diesel engines, inparticular under unfavourable operatingconditions. These occur for instance afterlonger idling periods following full-loadoperation, which frequently causes anejection of oil into the exhaust tract and

blue smoke emission upon acceleratingagain.

Fig. 3

Fig. 4

Copperplated rings are also well protectedagainst corrosion and provide some pro-tection against the formation of scumarks during the running-in process. Thecopper has a certain dry lubricating e ectand, consequently minimum dry-runningproperties during run-in.

However, these surface treatments do nota ect the function of the rings. The colourof piston ring does not make any di erence

to its quality.




1.6 | Functions and characteristics

1.6.1Tangential tensionPiston rings have a larger diameter in theiruntensioned condition than when tted.This is required to apply the requested all-side contact pressure in the cylinder bore.

The measurement of the contact pressurein the cylinder bore is quite di cult inpractice. The diametral load which pressesthe ring against the cylinder wall is thereforedetermined with the aid of a formula fortangential force. The tangential force is theforce requir ed to contract the joint endsuntil the joint clearance is achieved (Fig.1). The tangential force is measured bymeans of a exible steel strip which islaid around the ring. This strip is thencontracted until the speci ed jointclearance of the piston ring is achieved.The force can then be read on the force-measuring device. The measurement ofoil control rings is always carried outwhen the expander spring has beeninserted. To ensure precise measurements,the measurement set-up is subjected tovibrations to enable the expander springto adopt its normal shape behind the ringbody. The design of three-piece steel-railexpander rings also requires the ringpackage to be axially xed, because thesteel rails would otherwise escape tothe side and render the measurement

impossible. Figure 2 shows a diagramof tangen tial force measurement.

Fig. 1

Fig. 2

F

F t

F t

Important note:Piston rings lose their tangential tensiondue to radial wear caused by mixed frictionor longer service life. Measuring theirtension is only meaningful on new rings onwhich the cross section is still complete.



Functions and characteristics | 1.6

1.6.2Radial pressure distributionThe radial pressure depends on the modulusof elasticity of the material, the free gap inthe untensioned condition and, not least,on the cross section of the ring. There aretwo main distinguishing features for radialpressure distribution. The basic feature isthe symmetric radial pressure distribution(Fig. 3). This occurs mainly on multipieceoil control rings consisting of a exiblering carrier or steel rails with a relativelylow internal stress. The expander springarranged behind presses the ring carrieror the steel rails against the cylinder wall.The radial pressure acts symmetricallyas a result of the expander spring whichsupports itself in its compressed condition(i.e. when installed) against the rear of thering carrier or the steel rails.

On four-stroke engines, the symmetricradial pressure distribution is no longerused for compression rings. Instead, apear-shaped distribution (positive oval) isapplied to counteract the tendency of thering joint ends to utter at higher enginespeeds (Fig. 4). Ring utter always beginsat the joint ends and is then induced alongthe entire ring circumference. Increasingthe pressure force at the joint end counteractsthis, since the piston rings are pressedmore intensely against the cylinder wall

in this section and, as a result, the ringutter is e ciently reduced or stopped.

Fig. 3 - Symmetric radial pressure distribution

Fig. 4 - Positive oval radial pressure distribution



1.6.3Contact pressurereinforcement due tocombustion pressureThe contact pressure reinforcement resultingfrom the combustion pressure to which thecompression rings are subjected duringengine operation is, however, moreimportant than the internal stress of the

piston rings by far.

Up to 90% of the total pressure forceapplied by the rst compression ring isgenerated by the combustion pressureduring the combustion cycle. As shownin Fig. 1, the pressure is applied behindthe compression ring pressing them evenmore against the cylinder wall. Thecontact pressure reinforcement mainlyacts on the rst compression ring and,to a reduced extent, also extends to the

second compression ring.

Fig. 1

The gas pressure for the second pistonring can be controlled by varying the jointclearance of the rst compression ring.If the joint gap is slightly enlarged, forinstance, more combustion pressure willreach the rear of the second compressionring, also resulting in an increasedpressure force. With a higher number ofcompression rings, there is no contactpressure reinforcement from the secondcompression ring on due to the gaspressure from the combustion.

Mere oil control rings function due to theirinternal stress. Due to the speci c shapeof the rings, the gas pressure cannot actas a pressure ampli er in this case. Thedistribution of forces on the piston ring

also depends on the shape of the pistonring sliding surface. If taper faced ringsand ball-shape compression rings areused, the gas pressure also arrives in thesealing gap between piston ring slidingsurface and cylinder wall and acts againstthe gas pressure applied behind the pistonring (see chapter 1.3.1 Compressionrings).

The axial pressure force which is applied

to the compression ring at the lowergroove side is, however, only generated bythe gas pressure. The internal stress of therings does not act in the axial directionat all.


Important note:During idling, there is less contactpressure reinforcement of the rings bythe gas pressure because the combustionchambers are not properly lled. Thisis especially noticeable on diesel engines.Engines that idle for extended periodshave an increased oil consumptionbecause the scraper e ect is compromisedby the lack of any support through gaspressure. Frequently, engines then ejectblue clouds of oil out of the exhaust pipewhen accelerating after a longer idlingperiod; this is because oil was able togather in the combustion chamber andexhaust tract and is not burnt until thedriver accelerates again.



Fig. 3

1.6.4Speci c contact pressureThe speci c contact pressure dependson the ring tension and the contact areaof the ring on the cylinder wall (F A).If the speci c pressure force is to bedoubled-up, two options are available:either one doubles the ring tension or onehalves the rings contact surface in thecylinder. As can be seen in the illustration,the resulting force (speci c pressure force= force area) acting on the cylinder wallis always the same, although the ringtension has been doubled or halved, asthe case may be.

On new engines the trends is towards lowerring heights because the objective is toreduce friction in the engine. However, thiscan only be accomplished by reducing thee ective contact area between the ringand the cylinder wall. If the ring height isreduced by half, the piston ring tension,and consequently the friction, is reducedby half as well. Since the remaining forceacts on a smaller area, the speci c contactpressure on the cylinder wall (force area)with half the area and half the tensionremains as great as with double the areaand double the tension.

Fig. 2

AttentionThe ring tension alone cannot be used forassessing the contact pressure and thesealing behaviour. When comparing pistonrings, it is thus always necessary toconsider the size of the sliding surface.

F

F




A basic prerequisite for the properfunctioning of the piston rings is that therings can freely rotate in their grooves.If the piston rings seized in their grooves

they could neither seal nor dissipate theheat. The joint clearance, which must bemaintained even at operating temperature,ensures that the circumferential dimensionof the piston ring always remains smallerthan the circumference of the cylinderas a result of its thermal expansion. Ifthe joint clearance were nulli ed by thethermal expansion, the joint ends of thepiston ring would be pressed against eachother. If further pressure were applied,the piston ring would even have to bendto compensate the change in lengthcaused by heating. Since the piston ringcannot expand in its radial direction dueto the thermal expansion, the change inlength can only be compensated in theaxial direction. Figure 2 shows how thering will be deformed whenever the spacein the cylinder bore is insu cient.

Fig. 2

1.6.5Closed gap clearanceThe closed gap clearance is an essentialconstructional feature for ensuring theproper functioning of piston rings. It canbe compared to the valve clearance ofintake and exhaust valves. When everthe components are heated, the naturalthermal expansion causes an extensionof the length and/or the dia meter.Depending on the temperature di erencebetween ambient and opera tingtemperature, more or less cold clearanceis necessary to ensure proper functioningat operating temperature.

100 C

0 C

Fig. 1




Fig. 3

The following calculation shows the change in the circumferential length of the ring atoperating temperature using the example of a piston ring with a diameter of 100 mm.

Example of calculation:

Cylinder diameter d 100 mmAmbient temperature t 1 20 COperating temperature t 2 200 CCoe cient of linear expansion of cast iron 0.000010

In this example, a joint clearance of atleast 0.6 mm is required to ensure properfunctioning. However, not only the pistonand the piston rings expand, the diameterof the cylinder bore also increases whenheated at operating temperature. This isthe reason why the gap clearance canbe slightly smaller again. However, thecylinder bore expands far less as a resultof the thermal expansion than the pistonring. On the one hand, the structure of thecylinder block is more rigid than that ofthe piston; on the other hand the cylindersurface does not heat up nearly as muchas the piston with its rings.

Furthermore, the diameter expansion ofthe cylinder bore due to the thermalexpansion is not uniform along the entirerunning surface of the cylinder liner.Thecombustions heat introduction meansthat the cylinder expands more in theupper area than it does in the lower area.The uneven thermal expansion of thecylinder bore will therefore result in avariance in the cylinder shape which willbe slightly coneshaped (Fig. 3).

Change in length of piston ring at operating temperature

l = l1 tl = l1 (t 2 - t1 )

l = 314 0.000010 180 = 0.57 mm

Circumference of piston ring

U = d

U = 100 3.14 = 314 mmU = l1




1.6.6Piston ring sealing facesPiston rings do not only seal on the slidingsurface but also on the lower ank. Thesealing e ect on the sliding surface isresponsible for sealing the ring towardsthe cylinder wall; the lower groove sidetakes over the sealing of the rear end ofthe ring. Therefore, the ring requires notonly good contact towards the cylinderwall, but also good contact to the lowergroove side of the piston. If this contactis not given, oil or combustion gasesmay ow behind the ring.

With the aid of these illustrations, it iseasy to imagine that, due to wear (dirtand long service life), the sealing on therear end of the ring is no longer warrantedwhich causes an increased transfer of gasand oil through the groove. It is thereforea hopeless venture to t worn ring grooveswith new rings. The unevenness on thegroove side will no longer seal against thering, and when the groove has expandedin height, it provides more space for ringmovements. The fact that the ring is notproperly guided in the groove due toexcessive height clearance makes it mucheasier for the ring to lift o the grooveside, to pump oil (Fig. 2 and 3), and also

causes ring utter and a loss of thesealing e ect. Furthermore, excessiveconvexity on the sliding surface of thering occurs, making the oil lm too thickand causing increased oil consumption.

Fig. 2 - Intake cycle

Fig. 3 - Compression cycle

Fig. 1




1.6.7Throttle gap and blow-bySince - due to the design - it is not possibleto achieve a 100% gas sealing with thepiston rings used in engine manufacture,so-called blow-by gases are producedduring engine operation. Combustiongases escape through the tiniest sealinggap into the crankcase, past the pistonand piston rings. During this process, thevolume of blow-by gases is determined bythe size of the gas leakage area resultingfrom the joint clearance and half the pistonrunning clearance. In contrast to graphicsshown, the gas leakage area is, in reality,really tiny. As a rule of thumb for themaximum blow-by gas emission, about1 % of the taken-in air quantity shouldbe anticipated. Depending on the pistonring position, a greater or lesser volumeof blow-by gases is produced duringoperation. If the joint clearances of the

rst and second compression ring arecongruent in the piston ring grooves,the volume of blow-by gases is slightlygreater. This occurs in operation at regularintervals, since the rings rotate in theirgrooves with quite a few revolutions perminute. If the joint clearances are placedexactly opposite each other, it is evidentthat the blow-by gas has a longer way topass through the sealing labyrinth, thuscausing less gas losses. Blow-by gas

arriving in the crankcase is returned tothe intake air system via the crankcaseventilation and conveyed to the combustionprocess. The reason for this is the harmfulproperties of the gases. These are renderedharmless thanks to recombustion in theengine. Crankcase ventilation is requiredbecause excess pressure in the crankcasewould result in increased oil leakage onthe radial shaft sealing rings of the engine.Increased blow-by gas emissions indicate

Fig. 4

either considerable wear of the pistonring after long running time, or the pistoncrown already shows signs of crackingallowing combustion gases to pass tothe crankcase. But incorrect cylindergeometry (see Chapter 2.3.5 Cylindergeometry and roundness) will also resultin increased blow-by gas emissions.

On stationary engines or test-benchengines, the blow-by gas emission isregularly measured and monitored andused as a warning indicator for theoccurrence of engine failure. Wheneverthe measured blow-by gas volume exceedsthe maximum admissible value, the engine

will shut-o immediately. Consequently,severe and costly engine failures can beprevented.




1.6.8Ring height clearanceThe ring height clearance (Fig. 1) is not a result of wear in the ring groove. Theheight clearance is a functional dimensionthat is vital if the piston rings are tofunction properly. The ring height clearanceassures that the rings can move freely intheir piston ring grooves (see also Chapter1.6.11 Piston ring movements). The ringheight clearance must be great enoughto ensure that the ring does not jam atoperating temperature and that su cientcombustion pressure can stream into the

1.6.9

Ring twistingInside steps or inside bevels on pistonrings cause twisting in their tensioned(installed) condition. Once they have beendisassembled and are untensioned thereis no twisting (Fig.2), and the ring lies atin the ring groove. If the ring has been

Fig. 2 - Untensioned ringsTwisting without efect

Fig. 3 - Positive twist Fig. 4 - Negative twist

Abb. 1

groove to pass behind the ring (seealso Chapter 1.6.3. Contact pressurereinforcement due to combustionpressure). Conversely, the ring heightclearance must not be too large, asthe ring will then exhibit reduced axialguidance. This increases the tendencyto ring utter (Chapter 2.6.7 Ring

utter) and even to excessive twisting,which causes inappro priate wear ofthe piston ring (excessive convexity

of the ring sliding surface) andincreased oil consumption (Chapter1.6.6 Pistonring sealing faces).

installed - thus tensioned - it gives way tothe weaker side, which is where there is nomaterial due to the inside bevel or insidestep. The ring becomes twisted.Depending on the position of the bevel orthe step on the lower or upper edge, thisis refer red to as a positively or negativelytwist ing piston ring (Fig. 3 and 4).




Ring twisting under operating conditionsOn positive and negative twisting rings,twisting is e ective as long as there is nocombustion pressure acting on the ring(Fig. 5). Once the combustion pressurehas reached the ring groove, the pistonring is pressed at onto the lower grooveside causing an improved oil consumptioncontrol (Fig. 6).

Positively twisting rectangular and taperfaced rings present e cient oil scrapingbehaviour. However, in the event of frictionon the cylinder wall occurring during thedownstroke of the piston, the ring may liftslightly o the lower groove side, allowingthe oil to enter the sealing gap and thuscontributing to oil consumption.The negatively twisting ring seals the ringgroove at the outer lower ring ank and atthe inner upper ank thus obstruc ting theentrance of oil into the groove. For thisreason, oil consumption can be positivelyin uenced using negative twisting rings,in particular at part-load operation andvacuum in the combustion chamber(overrun condition).On negatively twisting taper faced rings,the angle on the sliding surface is approx.2 bigger than on normal taper facedrings. This is required because the angleis partly compensated by the negativetwisting.

Fig. 5 Fig. 6




1.6.10Form matching capabilityBy form matching capability we understandhow well the ring can adopt the shape of acylinder wall to achieve an e cient sealinge ect. The form matching capability of aring depends on the elasticity of the ring orthe ring body (two-piece oil control rings) orthe steel rails (multi-piece oil control rings)and on the contact pressure of the ring/ringbody against the cylinder wall. In the process,the form matching capability is the betterthe more elastic the ring/ring body is andthe higher the contact pressure. Large ringheights and big ring cross sections presenta high sti ness and, due to their higherweight, cause higher inertia forces duringoperation. As regards the form matchingcapability, they do worse than rings withlow ring heights and small ring crosssections and consequently lower inertiaforces.

Excellent form matching capabilities canbe found on multi-piece oil control ringssince these have an extremely exible ringbody or steel rails without need to complywith the requirement for high tension.As described in this brochure, on multi-piece oil control rings the pressure forceis applied by the associated expanderspring. The ring body or even the steel railis extremely exible and adaptable.

Fig. 1

An e cient form matching capability is ofparticular importance if shape devia tionscause cylinder ovalities and unevennesswhich are a result of both distortions(thermal and mechanical) and of machiningand tting errors. See also Chapter 2.3.5Cylinder bore geometry and roundness.




1.6.11Piston ring movementsRing rotationIn order for piston rings to run-in andseal properly, they must be able to rotatefreely in their grooves. The ring rotationis the result of the honing pattern (crossgrinding) on the one hand, and the rockingmovement of the piston at the top andbottom dead centre on the other hand.Flatter honing angles cause fewer ringrotations whereas steeper angles resultin higher ring revolution rates. The ringrotation also depends on the enginespeed. 5 to 15 revolutions per minuteare realistic gures to get an idea of thedimension of the ring rotation. On two-stroke engines, the rings are securedagainst rotation. As a result, both ringrotation and rebounding of the joint endsinto the gas channels are prevented.Two-stroke engines are mainly used inmotorcycles, gardening machines andthe like. The irregular wear of the rings,a possible coking in the ring grooves anda restricted service life due to inhibitedring rotation is tolerated in this regard.In any case, this type of application isdimensioned for a shorter service lifeof the engine from the outset. Therequirements made on a normal four-stroke vehicle engine that is in road use,are higher by far where the mileage is

concerned.

The twisting of the ring joints by 120 toeach other is only intended for facilitatingthe start of the new engine. After thisperiod, any conceivable position of thepiston ring inside the ring groove ispossible, provided the rotation is notinhibited by design (two-stroke engines).

Axial movementIdeally, the rings rest on the lower grooveside. This is essential for the sealingfunction, since the rings not only seal atthe piston ring sliding surfaces but alsoat the lower ring anks. The lower grooveside seals the ring against gas or thepassage of oil at the rear of the ring. Thesliding surface of the piston ring sealsthe front end towards the cylinder wall(also see Chapter 1.6.6 Piston ring sealing

faces).

Due to the up- and downstroke of the pistonand the direction reversal, the rings arealso exposed to inertia forces, causingthe rings to lift o the lower groove side.An oil lm inside the groove diminishesthe lifting e ect of the piston rings fromthe lower groove side caused by thesecentrifugal forces. Di culties mainlyarise if the ring grooves are expanded bywear and there is, excessive ring height

clearance as a result. This causes the ringto lift f rom its contact point on the pistonand also ring utter, mainly starting atthe joint ends. This provokes a loss of thesealing capacity of the piston ring andhigh oil consumption. This occurs mainlyduring the intake cycle whenever the ringslift o the groove base and oil is takeninto the combustion chamber past the rearend of the ring as a result of the pistondownstroke and the subsequent vacuum.During the remaining three cycles, thepressure arriving from the combustionchamber presses the rings onto thelower ank.




Ring twisting

As can be seen on the picture, inertiaforces, ring twisting and ring heightclearance put the rings into motion. Asdescribed in Chapter 1.5.6 Ballshapedsliding surface shapes, the piston ringswill adopt a ball-shaped shape after awhile if they are not already producedwith a ball-shape.

Radial movementThe rings do not in fact reciprocateradially, but rather the piston changesfrom one cylinder wall to the other as aresult of the reversal being performedinside the cylinder bore. This occurs bothin the upper and lower piston dead centre.The rings are thus moved radially insidethe piston ring groove. This results inan oil carbon layer grating as it forms(particularly for keystone rings), and in

a connection to the compound rest for ring rotation.

Fig. 1 Fig. 2

Fig. 3 Fig. 4




Installation and service | 2

Fig. 6

2.1Assessment of usedcomponentsAs an integral part of a sealing systemconsisting of pistons, cylinder bore,engine oil and piston rings, the pistonrings can only perform their task to theextent permitted by the functions of theother components. If the e ciency of

one sealing component is reduced, forinstance by wear, the overall e ciencyof the sealing system will be lowered asa result.

The re-use of already run interactingsliding parts of piston rings (pistons andcylinder bore) should be done usingcommon sense and expertise. The sealingsystem is only as e ective as its weakestcomponent. It is not practical or evensensible, to attempt to recondition an

engine by only changing the piston rings.If the rings are worn, one can also assumethat the interacting sliding parts of thepiston ring are also worn. Simply replacingthe rings while reusing a worn piston ora worn cylinder liner will not yield thedesired results. Remedying drops in poweror excessively high oil consumption istherefore a pretty hopeless venture andwill, if anything, bring only temporarysuccess. The causes on which thiscircumstance is based are described,

among others, in Chapter 1.6.6 Pistonring sealing faces.

Fig. 5



0 , 1 2

AttentionThe wear dimension refers to the outeredge of the ring groove to be measured,i.e. it must be impossible to insert thefeeler gauge with a thickness of 0.12 mmbetween piston ring and ring groove,as shown in Figure 2. Otherwise the ringgroove is regarded as already worn.

2.2.1Measurement andassessment of ringgroovesWhenever new piston rings are to be tted on used pistons, the ring height clearancewill decide on whether the piston can bereused. The piston ring concerned isinserted into the cleaned ring groove and

measured with a feeler gauge, as shown inFigure 6 (Page 39). If a new piston ring isto be measured in a used piston, it isbetter to use the method shown than to tthe piston ring onto the piston. Repeatedly

tting and removing the piston ring onand o the piston may cause a materialdeformation of the piston ring and a ectits functions.

Fig. 1

Ring height clearance (mm) Usability of the piston

0.05-0.10

Piston can be reused without

any problem.

0.11-0.12 Caution is strongly advised.

> 0.12The piston is worn and mustbe replaced.

2.2 | Assessment of used pistons



Assessment of used pistons | 2.2

It is not possible to examine the ringheight clearance of keystone rings whenthey are in their tted and untensionedcondition. Due to the keystone shape,the correct ring height clearance in thekeystone groove is only achieved afterthe piston ring has been compressed tothe cylinder size or mounted into thecylinder bore. As a result, measurementis almost impossible.

For this reason, the inspection must belimited to visually inspecting the groovefor wear (Fig. 2).

Test method during production

Fig. 2

Fig. 3

Nominal Dimension



2.3.1High-polished runningsurfaces of the cylinderliner (grey cast ironcylinder)Bright, mirror- nish cylinder surfaces without any more honing grooves areeither the result of natural wear after along running time, or caused by dirt andmixed friction after a short running time.

2.3.2Locally restricted brightspots due to cylinderdistortionsCylinder distortions cause out-ofroundnessat certain point inside the cylinder bore(Fig. 1). The position of the bright spotis identical with the point of origin of thedistortion. The piston rings pass theseconstrictions and remove material there.Insu cient lubrication and mixed friction

are caused by the constriction wheneverthe piston ring slides over the raised spotand consequently touches the cylinder wall.

The fact that all honing grooves have beenremoved due to wear is a sure indicationthat a cylinder bore is worn. Remeasuringusing appropriate measuring equipmentis no longer necessary. In any event,such cylinder bores should be replaced(cylinder liners) or newly bored and honed(engine blocks).

Causes:

Thermal distort ion due to local over-heating which is caused by poor heattransfer (contamination) to the coolingagent.

Non-observance of specified tighteningtorques, using wrong O-rings or otherdistortions as a result of tightening.

Remedies: Thorough cleaning and, if necessary,

refinishing of the cylinder counterboreof wet and dry cylinder bores.

Exact compliance with tighteningspecifications when installing thecylinder head.

Regular cleaning of the cooling finson air-cooled finned cylinders

Ensuring the specified function of thecooling system (circulation speed,cleanness).

Using the prescribed sealing rings(dimensions, material composition).

Fig. 1

2.3 | Assessment of used cylinder bores

Locally restricted bright spots on thecylinder sliding surface after a comparablyshort running time (the honing pattern inthis area is also completely removed) areevidence that the area where the brightspot has been formed has been exposedto mixed friction, resulting in increasedwear on the cylinder wall. Such locallyrestricted bright spots are mainly causedby two factors.



Assessment of used cylinder bores | 2.3

2.3.3Bright and polished spotson the upper cylindersection (Bore Polishing)There are bright spots in upper sectionof the running surface of the cylinderliner (Fig. 2) travelled over by the pistontop land. These spots are caused byhard deposits on the top land due to

irregular combustion, poor oil qualityor low combustion temperatures, causedby frequent idling periods or part-loadoperation. The carbon layer (Fig. 3) resultsin abrasive wear on the cylinder wall,an interrupted oil lm, mixed friction,increased piston ring wear andconsequently in high oil consumption.

Remedies: Proper engine operation. Using the specified oil qualities. Using branded fuels. Proper maintenance, inspection and adjustment of fuel injection system.

Fig. 2

Fig. 3



2.3.4Top ring reversal bore wearTop ring reversal bore wear (Fig. 1) appearsat the reversal points of the piston ringsat the top and bottom dead centre aftera longer runtime. The piston speed isreduced in this section, and even stopsbrie y at the reversal point. This reducesthe lubricating e ect, since the piston ringis no longer oating on the oil lm due tothe lack of relative speed to the cylinderwall and comes into metal contact with thecylinder wall. The example of a water skiermakes this clearer. As soon as the speedof the boat is inade quate the water skiersinks into the water.

X

Figures 2 and 3 show what happens whena new piston is installed into a worncylinder bore. Since the new piston hasno ring groove wear at all and pistonrings still have sharp edges, the pistonring edge hits against the wearing edgeof the cylinder. High mechanical forces,excessive wear, piston ring utter andhigh oil consumption are the consequence.

Fig. 1

Fig. 2 Fig. 3

Due to design, wear on the top ringreversal bore is most pronounced inthe piston ring reversal section closeto the top piston dead centre, becausethe cylinder surface is exposed to hotcombustion at that point which a ectslubrication.

The extent of top ring reversal bore wearis decisive on whether the cylinder lineror the engine block may be reused or not.

If the top ring reversal bore wear exceedsthe values speci ed in the chart, thecylinder liner must be replaced or theengine block rehoned. If the cylinder boreis worn to a similar extent in anothersection, the wear dimensions speci edbelow are also applicable.

Engine designTop ring reversalbore wear limit X

Petrol engines 0.1 mm

Diesel engines 0.15 mm





2.3.5Cylinder geometry androundnessPerfect cylinder geometries are theprerequisite for optimal piston ringsealing. Deviations from the cylindershape, out-of-roundness, dimensionalfaults and distortions in the cylinderbores result in sealing problems at the

Out-of-roundness in the bore geometry issubdivided into di erent orders. A perfectcylinder bore without any out-of-roundnessor form deviations in axial direction is aso-called rst order bore (Fig. 4). Ovalbores frequently due to the machiningfaults or poor heat dissi pation, are calledout-of-roundness of second order (Fig. 5).

Triangular third order out-of-roundness(Fig. 6) mostly results from a superpositionof second and fourth order distortions.Fourth order out-of-roundness (Fig. 7),thus square imperfect shapes, are usuallycaused by distortions arising fromtightening the cylinder head bolts.

The size of the out-of-roundness can bebetween zero and several hundredth of amillimetre. Due to small piston tting orpiston running clearances, as exist in

various engines, distortions of more thana hundredth millimetre (0.01 mm) mayconsequently already cause problems.Piston rings can only reliably seal smallsecond order out-of-roundness , i.e.slightly oval cylinder bores and minorkeystone shapes in axial direction.Third and fourth order out-of-roundness,as frequently caused by distortions dueto tightening and/or machining faults,rapidly make piston rings reach the limitsof their sealing function. In particular on

more recent piston designs with pistonring heights close to one millimetre oreven less, sealing is becoming increasingly

problematic. The constructional reductionof piston ring heights is intended fordecreasing frictional losses in the engineand thus reducing the fuel consumption.Since the contact areas to the cylinder wallof such rings are smaller, it is essentialthat the piston ring tension is reduced aswell. Otherwise, the speci c surface

pressure of the ring would become toogreat and the tribological propertieswould deteriorate. However, if the boregeometries are correct, this constructionalreduction of the piston ring tension has nonegative e ect. The rings seal very well,cause only minor frictional losses andhave a long service life. But on out-of-round distorted cylinders, the rings cannotor only very slowly adapt to the cylinder wallas a result of the low piston ring tension,and consequently, cannot ful ll their proper

sealing function.

Classi cation of out-of-roundnesson cylinders

Fig. 7

Fig. 6

Fig. 5

Fig. 4

piston ring and in an increased passage ofoil into the cylinder, increased blow-bygas emission, temperature and e ciencyproblems, premature wear and - notleast - in piston damage.



2.3.6Causes of out-of-roundness and distortionson cylinder boresOut-of-roundness and distortions incylinder bores may be caused by thefollowing: Thermal distortions caused by poor

heat dissipation as a result of faults

in the coolant circulation or due toclogged, oily cooling ns and/or

1. Bolt force of cylinder head bolts

2. Pressure force of cylinder head andcylinder head gasket

3. Cylinder bore deformation(overexaggerated representation)

The gure shows a fourth order cylinder distortion which frequently results due to the design whenthe cylinder head bolts are (even properly) tightened.

Fig. 1

ventilation problems on air-cooledengines. Local overheating on thecylinder sliding surface leads toincreased thermal expansion in thisarea consequently causing deviationsfrom the ideal cylinder shape.

Constructional thermal distortionsresulting from varying thermalexpansion during engine operation.

Thermal distortions resulting frompoor lubrication and cooling during

cylinder machining.

Out-of-roundness caused by excessivemachining pressures or by using wronghoning tools.

Distortion due to tightening on cylinderbores due to inaccuracies in shape andincorrect screw tightening.

1. 2.

4.

A

A

A

A


3. A-A




2.3.7Re nishing used cylinderboresWhen pistons or piston rings are replaced,so-called hone brushes or spring-loadedhoning stones (Fig. 2 and 3) are used inpractice. However, this has actually verylittle to do with honing. The more or lessworn running surface of the cylinder liner

is only cleaned and slightly roughenedduring this process, which does not bringabout any improvement of the cylindergeometry. Since the grinding tools arespring-loaded, they follow every out-of-roundness and distortion exactly withoutimproving the cylinder geo metry. As aresult of the low contact pressure, it isnot possible to achieve any meaningfulroughness which could contribute toimproving lubricating e ects. The frictionalresistance for the new piston rings is

slightly increased so that they can adaptmore quickly to the cylinder wall. But thewear on the cylinder surface cannot beeither undone or improved. If the pistonrings are worn, experience shows that thecylinder wall is worn to the same extent.The improved appearance of the cylinderbore should not hide the fact that it ismore like cosmetic operation than anexpedient reconditioning or repairmethod.

Fig. 2 Fig. 3



2.4 | Assembly of pistons and piston rings

The most severe piston ring problemsand damage arise when the rings are notproperly tted onto the piston. In thisprocess, the piston ring experien ces thegreatest mechanical strain. The contourand radial pressure distribu tion achievedduring production su ers due to improper

tment onto the piston to such an extentthat the required sealing function isfrequently achieved only f ractionallyor not at all.

A piston ring should only be expandeduntil the inside diameter can be slippedover the outer diameter of the piston.Any further expansion will result in ringdistortion, in particular at the back (Fig. 1),causing considerable sealing problems inits installed condition.

Fractures, coating separation (in particularon molybdenum- lled rings), reducedpressure forces at the back of the ring up

to generated crescent-shaped gaps (Fig. 2)impair the proper functioning of the ring ormake it fail completely.

Fig. 1

Fig. 2

Attention

Never bend up piston rings to increasetheir tension! If the joint ends are pulledapart, the ring will only bend at one point -at the back. The ring tension cannot beincreased in this way. Far from it. If thering is excessively bent or deformed, itloses its round shape and will never beable to seal properly.



Assembly of pistons and piston rings | 2.4

2.4.1Fitting and removingpiston rings

Clean used pistons thoroughly fromadhering dirt. Make sure that the ringgrooves are free of carbon and dirt.Use a drill or another suitable tool toclean the oil draining bores, if necessary.

Pay attention that no damage is caused

to the groove sides when removing thecarbon. The lower groove side is asealing face. Damage due to scratchesmay cause high oil consumption or anincreased blow-by gas emission duringengine operation.

It is essential that piston ring pliers areused for tting and removing pistonrings. Other devices, such as a wireloop or screwdriver, damage the pistonring as well as the piston.

Never t the rings manually (except

steel rail oil control rings). There is notonly the risk of ring fracture, distortionand overstretching, but also the dangerof injury should a ring break or due tosharp ring edges.

Fig. 3

AttentionIf the piston ring is tted quickly byhand and without breaking, this maybe evidence of the skills of the mechanic,but it will usually result in damage to thepiston rings when they are tted.

Piston Ring Service Tool SetArticle No. 50 009 913



Never t a ring onto the piston as shown(Fig. 1). If the ring is deformed and nolonger lies at in the groove, it can nolonger rotate in it, wears on one side orwill no longer seal properly. Even worseis, however, the peeling or partial fractureof the molybdenum coating of a ring.If the loss of the sliding layer does notalready occur during installation, itwill appear at the latest during engineoperation. The sliding layer peels o ,

which results in damage to piston andcylinder surface. The piston seizesin the cylinder bore because hotcombustion gases are blown betweenpiston and cylinder wall. The looseparts cause damage to the piston andcylinder sliding surfaces.

Avoid any unnecessary tting andremoval of the piston rings. The ringswill become slightly deformed duringeach tment. Do not pull-o rings f romalready pre-assembled pistons, for

instance to remeasure them. Stick to the tting sequence of therings. First mount the oil control ring,then the second and then the rstcompression ring.

Pay attention to the markings. Topmeans that this side must face the top,towards the combustion chamber. Ifyou are not sure or if there is no Topmarking, then mount the ring withthe inscription facing upward. Topdoes not mean that this is the rst

compression ring.

Fig. 1

Fig. 2




Check whether the rings can rotatefreely in the ring grooves.

Check whether the ring disappearscompletely in the ring groove along itsentire circumference, i.e. the slidingsurface of the ring should not protrudeover the piston skirt. This is essentialsince the ring function is not warrantedif there is no groove base clearance(incorrect ring tted or groove basecarbonised).

When mounting two-piece oil controlrings, always pay attention to theposition of the spiral expander (Fig. 6).The ends of the spiral expander mustalways be opposite the ring joint.

Fig. 3

Fig. 4 Fig. 5

Fig. 6




With three-piece rings, the correctposi tion of the expander spring isindispensable for ensuring the oilscraping function (Fig. 1 and 2). Priorto installing the piston, always checkthe position of the expander springson pistons with pre-assembled rings.The ends of the springs are in anuntensioned condition and may slideover each other. Both colour markingson the expander spring ends must be

visible (Fig. 3). If these are not visible,the spring is overlapping and the ringcan consequently not function properly.All ring joints of the three-piece oilcontrol ring (the two steel rails and theexpander spring) must be mounted withan o set of 120 to their counterparts.

Rotate the piston ring joints of the pre-assembled piston in such a way that thering joints are approx. 120 o set toeach other. This helps the pistons and/or the rings during the rst engine start.Reason: The compression at the rstengine start is slightly lower since thepiston rings have not yet been run-in.O setting the joint ends prevents toomuch blow-by gas from being producedduring the rst start of the engine andthe engine starts poorly.

Fig. 2Fig. 1

Fig. 3

Fig. 4




2.4.2Fitting the piston into thecylinder bore

Thoroughly clean the sealing surface ofthe engine block from sealing residues,if this has not been done during thereconditioning process.

Thoroughly clean all tap holes from anyadhering dirt, oil and coolant.

Carry out all cleaning work prior totting the pistons into the cylinderbores.

Apply a thin layer of fresh engine oil toall piston surfaces. Do not forget thepiston pin and conrod bearings.

Pay attention to the assembly directionof the piston (markings on the pistoncrown, valve pockets).

Clean the cylinder bore again using acloth moistened with engine oil.

Check your piston ring scu band for

damage and dents and remedy these orreplace the tool if necessary.

Take care during piston tting that thescu band or the conical assemblysleeve are positioned at on the cylinderhead sealing surface.

Fig. 5

Fig. 6

Piston Rings | 53




F

Do not use too much force to installthe piston. If a piston does not slidesmoothly in the cylinder bore it isessential to check the scu band. Theopening of the band in should notnecessarily be turned to correspond tothe joint ends of the rings.

Never install the piston in the enginewithout using a tool (risk of injury, riskof ring f racture).

If a hammer handle is used for instal-

lation, it is essential that only the deadweight of the hammer acts on the pistoncrown. Never use the hammer to drivethe piston forcefully into the cylinderbore. Even if the piston rings do notalready break during installation, theymay still be deformed and will not ful ltheir task properly.

Forceful installation will not onlydamage the rings but may also damagethe piston. This is particularly the casewith pistons in petrol engines . Their top

or ring lands can be extremely thin andeasily break partially or completely ifthey are hit. Power loss and early (andcostly) repairs will then result.

Prevent dirt and sand from falling intothe cylinder bore after the pistons havebeen installed. Place or insert cleancloths on/into the bores if necessary toavoid any penetration of dirt . This is inparticular recommended for workcarried out outdoors or in very dustyenvironments.

Fig. 1

Fig. 2




Initial start and running-in of engines | 2.5

2.5.1GeneralWhen speaking of running-in engines, onenormally thinks of all the moving partswhich have to adapt to each other. This isbasically correct, but it applies in particularto piston rings. Piston rings are componentswhich as a result of their task, are subjectedto the greatest stresses and not only haveto adapt to the surface of their associatedparts, but must afterwards seal perfectly.Piston rings are thus the componentsthat pro t the most from proper and thuse cient running-in. None of compo nentssupplied with pressure-oil has to withstandsuch great strain as piston rings.

There are quite di erent opinions bet weencustomers and technicians as regardsthe initial starting and running-in ofreconditioned engines. There is on theone hand the opinion that a run-in timebetween 500 and 1500 km is still required,and the second point of view is that therun-in time is not neces sary. The latterconclusion is also based, not least, on thestatements of some engine manufacturersthat do not require any speci c enginerunning-in. Both opinions are correct andjusti able and one ought to just di erentiatebetween brand-new and reconditionedengines.

Fig. 3



2.5 | Initial start and running-in of engines

2.5.2Running-in brand-newenginesThe most modern procedures areimplemented nowadays for producing newengines. The interacting sliding parts areproduced at such a high rate of precisionthat the processes that occurred earlier,during the engines run-in time, are

already anticipated during the specialmanufacturing process for the components.This is accomplished by speci c processes(for instance for running surfaces ofcylinder liners) as well as by precision-machining processes for the otherinteracting sliding parts. The processesused are primarily lapping processesto remove from the surfaces super neburrs and unevennesses that havebeen generated during the machiningprocesses. In former times, this adaptation

was left to the interacting sliding partswhich had to adapt to each other duringthe run-in time. However, this causedconsiderable material losses. Piston rings,for instance, already lost a considerablepart of their wear reserve during the rstoperating hours. Especially nowadays,when every milligram of exhaust emissionis cause for haggling, engines are soughtafter which keep their de ned fuel-consumption gures and, as a result,maintain their pollutant limit values,

right from the very start.

Fig. 1

An engine run-in during which the slidingsurfaces must rst adapt to each other byway of friction and above- average wear ishardly conceivable in modern enginemanufacture. The end user expects to geta mileage from the engine which is manytimes greater than anything considered asoptimal 25 years ago. A brand-new vehicleis subjected to a coldstart marathon rununtil it has passed the various logis ticscentres and transports and arrives at the

customer. An engine may frequently haveto perform up to 150 cold starts withoutreaching operating temperature betweenthese starts. In this context the shipmentto other countries and continents has to beconsidered. An engine that still had to berun-in would have a very bad start underthese circumstances.

Another reason for the relaxed running-inrules for brand-new vehicles is the factorthat, due to todays tra c density, it isalmost impossible to operate the vehiclesat their performance limits. Even onmotorways without any speed limit, it ishardly possible to reach the top speed orthe rated engine power of the vehicle fora longer period. It used to be possible forthe driver of a 30 kW vehicle travellingrapidly at a low maximum speed to drive his

vehicle for a long time at full load withoutany di culty even on a normal countryroad.



Fig. 2

Initial start and running-in of engines | 2.5

2.5.3Running-in reconditionedor repaired enginesIn contrast to brand-new engines, it isabsolutely essential to run-in reconditionedengines on which new cylinder liners havebeen used or the cylinder bores have beenbored to the next oversize and honed.In practice, an engine reconditioner

(depending on the machinery and equipmentavailable) cannot always work as preciselyand so free from contamination as duringinitial production in the manufacturing plant.

Used engines will not become as-new as aresult of reconditioning. Frequently, newand used parts are combined and, to savecosts, engines are not consis tently andcompletely overhauled. Running -in ismost necessary whenever cylinder bores,cylinder heads or crank shafts have been

refaced. In practice, it is not alwayspossible to achieve the same machiningparameters as during initial production,since the values are unknown or theavailable machines only allow for standardmachining processes. For these reasons,it is recommended that the followinginstructions for running-in reconditionedengines are observed.



2.5.4Running-in instructions forreconditioned engines Always run-in the engine on the road or

an engine test rig. Do not fully load the vehicle. Operate the engine up to max. 2/3 of its

maximum speed and changing theengine speed at intervals.

When driving, shift gears up rapidly,avoid low-speed driving conditions,and do not run the gears up to

aros de pistón

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