2009 · head gasket evolution the cylinder head gasket is arguably ... systems were largely...
TRANSCRIPT
Pg 1 Head & Block Decks & GasketsPg 6 Cylinder Bores & Piston Rings Pg 12 Valves & Valve Seats Pg 16 Cam Bores, Bearings &
Camshafts
2009APRIL
ince the days of sealingengines with asbestos, cork,rope and paper are, for themost part, ancient history,
new-age materials and designs haveelevated the critical role gaskets andseals play in the longevity of anengine. Finding the optimum sealingmaterial and design remain a chal-lenge many gasket manufacturers faceas engines are asked to do more.
Gaskets that combine high per-formance polymers with metal orfiber substrates are being used in moreand more engine applications.Furthermore, engines that are sealedusing these higher quality gaskets arelasting longer, performing better andwith less maintenance or chance ofthe dreaded warranty comebackclaim.
There are many considerationsengine builders must take intoaccount when selecting the best gas-kets for an application. With engineshaving higher performance require-ments, stricter emission regulations,and tighter design tolerances alongwith production cost considerationsand quality standards that are increas-ing, the last thing any engine builderwants to worry about is a gasket orseal failure.
It doesn’t matter if you’re buildinga Pro Stock or stock productionengine: the risks are simply too greatto use a subpar gasket or seal.
It’s more complicated to seal today’sengines because, depending on theapplication, modern engines areexpected to last well over 150,000miles without fail. Performanceengines are pumping out more powerthan ever. So keeping these enginessealed can be a tall order and creates ahefty job for engine gaskets and sealsto handle.
Engine seals and gaskets are sup-posed to prevent the leakage of oil,coolant and air between mating sur-faces, internal passages and the outsideof the engine. Seals and gaskets alsoprevent the entry of dirt and air intothe engine.
Smooth Operation How smooth is smooth enough? Youused to be able to tell by draggingyour fingernail across the surface of acylinder head or engine block. Andbesides, it didn’t really matter becausethe composite head gasket would fillany gaps that your equipment or tech-nique left behind.
But with MLS gaskets the require-ments have changed.To seal properly,a head gasket requires a surface finishthat is within a recommended range.This range varies depending on thetype of gasket.Too rough (or in somecases too smooth) and the gasket maynot seal properly and leak or fail.
According to surfacing experts, therecommended surface finish for a tra-ditional composite-style soft-facehead gasket in an engine with castiron heads and block is 60-120microinches Ra (roughness average).The recommended finish for the samehead gasket with an aluminum headon a cast iron block is typically 20-50microinches Ra.
For engines with MLS gaskets, theOEM surface finish recommendationsare even smoother, perhaps 20-30 oreven 7-15 Ra.
Accurately measuring the surfacefinish can be done using a profilome-ter, an electronic instrument that dragsa diamond-tipped stylus across thesurface to calculate its profile charac-teristics.The profilometer then showsvarious values for the surface includ-ing roughness average (Ra), averagepeak height (Rpk), average valleydepth (Rvk) and even waviness.
These critical surface finishesrequire high quality resurfacing equip-ment to achieve low Ra numbers.Extremely smooth finishes requirehigh quality resurfacing equipment toachieve really low Ra numbers. Itdoesn’t matter if you use carbide,cubic boron nitride (CBN) or poly-crystalline diamond (PCD) tool bits toresurface a head as long as you use thecorrect feed rate and speed – and theequipment is rigid enough to hold thecutter steady so the tool bit doesn’t lift
or chatter when it makes an interrupt-ed cut.
For example, a converted grindermay be able to mill heads and blocks.But the spindles and table drives inmany of these older machines cannothold close enough tolerances toachieve a really smooth, flat finish.One equipment manufacturer saidgrinding and milling machines thatare more than five years old are prob-ably incapable of producing consistentresults and should be replaced.
Most of the surfacing equipmentthat’s being sold to shops today hasbeen redesigned for high speedmilling with CBN and PCD. Themachines have been beefed up withmore powerful motors, heavier cast-ings, electrically driven ball screwtables, and tighter assembly tolerances.Some can hold machining tolerancesto .0001˝!
Doing the job right may be morechallenging than ever, but doing thejob wrong may cost you even more.
Head Gasket EvolutionThe cylinder head gasket is arguablythe most important seal in the engine.Since the very first internal combus-tion engines were produced, gasketdesigners have specified many materi-als to meet this difficult sealing chal-lenge.
Three Types of Head GasketsComposite – this technology is
used mostly on older stock engineapplications and in some performanceapplications. Typically these types ofgaskets are made from graphite andcan be more prone to blowouts thanMLS gaskets. In the aftermarket, com-posite gaskets are still required formany older applications and are alsomore forgiving of less than perfectsurface finishes.
Copper - A solid sheet of coppertypically requires machining for O-rings that place a piece of wire aroundthe circumference of the cylinder tobite into the copper.When used withO-rings, copper gaskets are extremely
S
www.enginebuildermag.com | PERFORMANCE PARTS & MACHINING TECH GUIDE 1
1-5 Gaskets/Decks 4/15/09 10:51 AM Page 1
durable. Copper gaskets are mostlyused in performance applicationswhere there is a lot of cylinder pres-sure. Recently some gasket companieshave started producing copper gasketswith integral sealing wires permittingtheir retrofit into engines without theneed to machine the engine block.
Copper and brass gaskets weresome of the earliest materials used butgave way to metal and asbestos com-posite gaskets in the 1950s, supersededin turn by composite metal andimpregnated fiber or graphite com-posites by the 1980s. However, thosesystems were largely overtaken by thedevelopment of the Multi-Layer Steel(MLS) gasket in Japan during the early1990s.
Multiple Layer Steel (MLS) –this type of head gasket typically con-sists of three layers of steel. Most mod-ern engines today are produced withMLS gaskets. The contact layers areusually coated with a rubber-likecoating such as Viton that adheres tothe cylinder block and cylinder headrespectively while the thicker centerlayer is bare, allowing for movement asif it were like a spring.
MLS gaskets remain the automo-tive industry’s preferred method ofsealing the cylinder head and block,
and is unmatched by any other sealingsystem to this point.Today, it is rough-ly estimated that 80 percent of newengines are designed with MLS gas-kets as standard equipment and furthergrowth is projected.
In the MLS gasket, multiple thinlayers of cold-rolled spring gradestainless steel are coated with 7-25microns of elastomeric material. Theresilient elastomer is essential to thestructure by providing micro-sealingof metal surface imperfections whileresisting aggressive combustion gases,oils and coolants at temperatures up to250°C or 480° F.
Better SealingThrough ChemistryGasket manufacturers use all kinds ofmaterials and designs to keep engineoil, coolant, vacuum and compressionseparated from each other. Throughthe use of computer aided design(CAD), finite element analysis (FEA)and polymer science, gasket engineerscan now more thoroughly understandmany facets of the joint they are try-ing to seal and are able to design a gas-ket with materials that will withstandthe pressures of the environment.
Materials such as Teflon, Viton,Graphite and other specialized materi-
als are taking the place of older, lesscompatible and less durable materials.The following list shows the types ofmaterials used in automotive applica-tions and the characteristics of eachtype.
Viton – Viton is a registered trade-mark of DuPont PerformanceElastomers and is commonly calledFluoroelastomer/ Viton (FPM). It isbest known for its ability to toleratehigh temperature environments(400°F), which makes it well suited forautomotive applications.
Viton can also easily withstandaggressive fuels and chemicals and hasexcellent mechanical properties aswell. This material is also resistant toabrasion and cuts. With a broad arrayof applications in the automotive,chemical processing and industrialmarkets,Viton fits many performancespecifications.
Neoprene Polychloroprene –Neoprene polychloroprene was the firstsynthetic elastomer able to resist oil andis still a popular material in a number ofautomotive applications. Neopreneremains a popular choice for manyautomotive applications that require areasonably priced, mid-performancepolymer with a good all-around bal-ance of performance properties.
2 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009Circle 102 for more information
1-5 Gaskets/Decks 4/15/09 10:51 AM Page 2
Nitrile (NBR) – NBR provides excellent resistance topetroleum oils and gasoline as well as mineral and vegetableoils. However, NBR has poor resistance to the swellingaction of oxygenated solvents such as acetone and ketones.It has good resistance to acids and bases except those hav-ing strong oxidizing effects. Resistance to heat aging isgood, which is a key advantage over natural rubber.
Higher acrylonitrile contents increase solvent resistancebut decrease low-temperature flexibility, and compoundingthis material to improve low-temperature flexibilitydecreases oil and solvent resistance.This material does notcrystallize on stretching and reinforcing materials arerequired to obtain high strength.With compounding, it ispossible to get a good balance between low creep, goodresilience, low permanent set and good abrasion resistance.Tear resistance and electrical insulation properties of NBRare inferior to natural rubber.
Silicone Rubber – Silicone rubber offers stability overa wide temperature range, resistance to harsh environ-ments, and long lasting performance exceeding manyorganic elastomers. Silicone rubber offers incredible resist-ance to extreme temperatures, being able to operate nor-
mally from minus 100°C to plus 250°C (212° to 482° F).When compared to natural rubbers, silicone has better fireresistant properties and is an excellent electrical insulator.Thermal stability means that properties such as volumeresistivity, dielectric strength and power factors are notaffected by changes in temperature.
Fluoroelastomer – FKM has been utilized by much ofthe European OEs and is the most successful coating mate-rial for long-term functionality critical sealing environ-ments such as automotive engines.Although nitrile rubber(NBR) has been widely used for MLS coatings in the USand Japan, the use of FKM is expected to grow rapidlyamong the OEs in both of these markets in next few years.
Head Gasket FailuresWith most OEMs choosing to use aluminum rather thaniron cylinder heads, the chance of a head gasket failure isincreased. Even though aluminum is lighter than iron, it hasa much greater thermal expansion rate, which in turn cre-ates more stress on the head gasket. Engine manufacturershave responded to this by adding a non-stick coating suchas Teflon or some other type of slick coating to the surface
of the head gasket.Most engine builders know that when a
head gasket fails, a whole range of problems canoccur, from compression loss to exhaust gasesbeing forced into the cooling system, leading tothe engine overheating and increased enginewear due to the oil being mixed with antifreeze.Coolant can leak into the cylinders, causing theexhaust to blow off steam and the steamycoolant mixture can damage the catalytic con-verter.
If a very large amount of coolant does this,hydrolock can occur, causing extensive enginedamage. Sometimes, all that may happen whena head gasket is blown is an eruption of steamfrom the tailpipe, though the engine may actand drive like normal.
If your customer suspects a blown head gas-ket you or your installer can investigate the sit-uation by checking the compression with apressure gauge, but the preferable method is aleak-down test, noting any indication of com-bustion gasses in the cooling system. Oil mixedwith coolant and excessive coolant loss with noapparent cause, or presence of carbon monoxideor hydrocarbon gases in the expansion tank ofthe cooling system can also be signs of head gas-ket problems.
Intake Manifold GasketsThe head casting-to-intake manifold joint iscritical because if you experience a failure itwill create a vacuum leak which will in turn
4 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009Circle 104 for more information
1-5 Gaskets/Decks 4/15/09 10:51 AM Page 4
create a poor running engine and anunhappy customer.
It is recommended that you checkthe head and manifold sealing surfaceswith a straightedge and machine themflat if any warpage is found. As withthe head/block seam, premium gasketsof graphite or with synthetic faces andperhaps a printed-on bead are com-monly available, and usually noretorquing is needed.
Another relatively new type ofmanifold gasket has a metal or plasticcarrier with rubber inserts or deposit-ed rubber beads.You have to try pret-ty hard to make a mistake whileinstalling these gaskets because thecarrier acts as a positive stop so therubber cannot be over compressed.These gaskets don’t require any sealanteither.
Also,V8 and V6 engines that havetabs or endstrips on the manifold gas-kets should be handled carefully. Thetabs are easy to dislodge while you’relowering the manifold into place.Youcan use a light coat of adhesive if youwant to try and keep them in place.
Intake manifold bolts should betightened in a pattern similar to thesequence used to torque the cylinderhead down.The goal here is to workfrom the center outward unless manu-facturer specifications say otherwise.Do not over-torque these bolts. Notonly will this save you from the disas-ter of a snapped bolt, but on someapplications it can prevent a head gas-ket failure. The pressure from theweight of the manifold can push theheads outward enough to reduceclamping force that could lead to ablown head gasket.
Valve Cover and Pan GasketsThe most basic type of gasket is thecut gasket, which are made by die-cutting sheets of fiber-reinforcedpaper-like material or groundcork/rubber to create the flange shapeof the cover. Cut gaskets are mostoften used in timing covers, valve cov-ers, oil pans, intake manifolds, carbure-tors, throttle bodies, water pumps,
thermostat housings, etc. Cut gasketsare also the least expensive to makeand allow the OEMs to use stampedsteel valve covers, timing covers andoil pans instead of more costly castaluminum.
Older style cork/rubber gasketsdon’t last forever and have a tendencyto leak, which is why most of thesegaskets are no longer used by OEMs.Most manufacturers use molded rub-ber or plastic carrier gaskets for theseapplications.The plastic carrier gasketis much more durable and can last aslong as the engine without failure.
Over-tightening these gaskets willoften crush or crack the gasket, whichthen leads to a failure down the road.
Some pan and cover gaskets haveextra features that allow them to beinstalled more easily than standardcork/rubber gaskets. Features include
grommets for lining up the holes andmetal carriers to make the gasketstiffer so it won’t flop all over whenyou try to line everything up.
In order for pan and cover gasketsto seal properly the surfaces must beclean, dry and as flat as possible, freefrom anything that might catch or tearthe gasket material. The surface mustalso be free of any old gasket residue.
Oil pan sealing innovations includemetal grommets in the boltholes orshouldered fasteners to guard againstover-torquing, and one-piece rubbergaskets.
For timing chain covers, thermostathousings, etc, a thin paper-type gasketis often used with a thin bead of agood sealant. Like all other parts of theengine, careful torquing proceduresare required with these gaskets. PPMG
www.enginebuildermag.com | PERFORMANCE PARTS & MACHINING TECH GUIDE 5Circle 105 for more information
1-5 Gaskets/Decks 4/15/09 10:51 AM Page 5
rying to classify theAmerican performancemarket into a single niche isan exercise in futility.Simplysaying “performance is alive
and well” can be proved by looking atthe scope of the performance pistonmarket.
Cubic inches is the name of the gametoday.The performance piston market isbeing driven by bigger bore blocks andstroker crankshafts.Top rings are runninghotter than ever before, thanks to highercombustion temperatures and pistonloads combined with rings that are relo-cated closer to the top of the piston forlonger stroke crankshafts.
With the performance piston marketbeing driven by a wider range of vari-ables than ever, piston ring suppliershave had to work hard to keep up aswell. Most performance pistons havesome type of coating or anodizingtreatment in the top ring groove to pre-vent the ring from welding itself to thepiston or pounding out in the groove.The top ring, in most cases,must also besteel or ductile iron, and faced or coat-ed with some type of wear-resistantmaterial such as moly, chrome, nitridingor one of the new “aerospace” vapordisposition or plasma spray coatings.
Moly is still the most popular facingmaterial for many piston rings becauseof its excellent wear and scuff resist-ance.Another material is tungsten car-bide (for hard liners), and chrome.Chrome-faced rings are still a goodchoice for abrasive environments likedirt tracks, and work best with castiron blocks and cylinder liners.Chrome rings don’t work well withchrome plated bores or hard facedcylinder bore liners such as those coat-ed with nickel/carbide coatings.
Thin is in, both at the beach and onthe track. Thinner rings generate lesstension and reduce friction, so ringdimensions have been shrinking inmany racing engines. Many top leveldrag racing and stock car enginebuilders go with compression rings asthin as 0.7 mm, and most Formula One
engines are now built with 0.6 mmrings.These skinny rings produce verylittle tangential load (about 1.5 pounds),which minimizes friction and allowsthe rings to conform more easily toany bore distortion. Many of theseengines are also using Napier-stylesecond rings, with a small notch in thebottom face of the ring to improve oilcontrol and sealing as the ring scrapesagainst the cylinder wall. The napierdesign is also used with a positive twistto improve its sealing characteristics,effectively helping to prevent detona-tion by keeping oil out of the combus-tion chamber.
Even at the Sportsman racing level,advancements in ring technology havebeen seen.While cast iron piston ringsremain popular with many budget-minded dirt track claimer motors aswell as many street performance andother racing applications, ductile ironor steel rings are usually required forhigher output engines. Ductile ironrings have roughly twice the tensilestrength of gray cast iron, and threetimes the fatigue strength. Steel rings,by comparison, have almost four timesthe tensile strength and fatiguestrength of gray cast iron.
For high boost turbocharged andsupercharged engines, and enginesusing large doses of nitrous oxide to addhorsepower, ductile iron or steel toprings are probably a must. Many racersprefer to use nitrided rings made fromsteel wire because the rings can handlehigh loads and thermal shock betterthan other materials. Nitriding pene-trates into the metal and won’t flake offlike other surface coatings.
Most performance pistons havesome type of coating or anodizingtreatment in the top ring groove to pre-vent the ring from welding itself to thepiston or pounding out the groove.The top ring, in most cases, must alsobe steel or ductile iron, and faced orcoated with some type of wear-resist-ant material such as moly, chrome,nitriding, or one of the new “aero-space” vapor disposition or plasma
spray coatings.Another reason for the smaller rings
in many performance engines is thatpistons are getting shorter. Longer con-necting rods with shorter pistonschange the combustion dynamics andprovide better angularity during thepower stroke.Shorter pistons also weighless, which means the engine can revhigher. But when the piston is shorter,the rings have to move up higher.Thismeans they have to be narrower,stronger and more heat resistant becausethe top ring is closer to the combustionchamber.
So what does this mean inside anengine? It means ductile iron and steelrings can survive in racing environ-ments that may be too demanding forgrey cast iron rings. Stronger ringsreduce the risk of ring breakage undersevere loads. Steel rings also show lessside wear and ring groove pound out.
Bore FinishRegardless of what kind of rings or lin-ers are used in a performance motor,rings usually work best when the cylin-der bores are given a plateau finish. Aplateau finish essentially duplicates a“broken-in” bore finish, so there is lessscrubbing and wear on the rings whenthe engine is assembled.What’s more, ifthe surface is finished correctly it willprovide plenty of flat, smooth bearingsurfaces to support the rings while alsoretaining oil in the crosshatch valleys tolubricate the rings.
The only exception to this is inmotors where there is a lot of bore dis-tortion. If the bores go out of roundwhen the head bolts are torqued down,the rings may not seat as well allowingincreased blowby and oil consumption.Thinner rings that can conform to thebore will work better in these kinds ofapplications, but it’s also a good idea touse torque plates when honing whenhoning the bores to simulate the distor-tion that occurs when the cylinderheads are installed.The other option isto go with a slightly rougher “peaked”finish to seat the rings.
T
6 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009
6-9 Bores/rings 4/15/09 10:53 AM Page 6
Most ring manufacturers recom-mend using a two- or three-step hon-ing procedure to achieve a plateau fin-ish. First, rough hone to within .003˝ offinal bore size to leave enough undis-turbed metal for finish honing. Forplain cast iron or chrome rings in astock, street performance or dirt trackmotor, hone with #220 grit silicon car-bide stones (or #280 to #400 diamondstones) to within .0005˝ of final size.Then finish the bores with a few strokesusing an abrasive nylon bristle plateauhoning tool, cork stones or a flexibleabrasive brush.
For moly faced rings in a street per-formance, drag or circle track motor,hone with a conventional #280 grit sil-icon carbide vitrified abrasive, then fin-ish by briefly honing to final size with a#400 grit vitrified stone or #600 gritdiamond stone (or higher), plateauhoning tool, cork stones or a brush.
For stock and street performanceengines with moly rings, an averagesurface finish of 15 to 20 Ra is typical-ly recommended. For higher classes ofracing, you can go a little smoother,provided you don’t glaze the cylinders.
For moly or nitrided rings in a per-formance motor, hone with #320 or
#400 vitrified stones, and finish with#600 stones, cork stones, a plateau hon-ing tool or brush.
If the cylinders are rough honedwith diamond, they can be finish honedwith a finer grit diamond, a fine-gritvitrified abrasive or a plateau honingtool or brush. Because diamond is aharder material and wears more slowlythan conventional abrasives, it cuts dif-ferently and may require more honingpressure. But many newer diamondstones now use a more friable bond thatstays sharp and doesn’t load up, allowingthe stones to cut smoother and leave arounder, smoother bore finish.
When using diamond-honing stonesinstead of vitrified abrasives, you gener-ally have to use a higher number grit toachieve the same Ra (roughness aver-age) surface finish.The actual numberswill vary somewhat depending on thebrand and grade of the stones.
Bristle style soft hones (plateau hon-ing tools) have mono-filament strandsthat are extrude molded with a fineabrasive material embedded in thestrands. The filaments are mounted indifferent types of holders for use withportable or automatic honing equip-ment.Another type of brush uses mold-
ed abrasive balls that are mounted onflexible metal shafts so the balls can eas-ily conform to the surface. Brushinghelps sweep away torn and folded metalon the surface while removing many ofthe sharp peaks to make the surfacesmoother.
When finishing the cylinders with abrush, only light pressure is required.The rpm of the brush should be similarto that which the cylinder was original-ly honed, and no more than 16 to 18strokes should be applied (some say 8 to10 strokes is about right). Too manystrokes with a brush may produce toosmooth a finish in a cast iron cylinderthat won’t retain oil. Reversing thedirection of rotation while brushinghelps to remove the unwanted materialon the surface.The end result should bea cylinder that provides immediate ringseal with little if any wear on the cylin-der wall or rings when the engine isfirst started.
With the right plateau honing tech-niques, you should be able to get thesurface down to an average roughnessof 8 to 12 Ra or less,with Rpk (relativepeak height) numbers in the 5 to 15range, and Rvk (relative valley depth)numbers in the 15 to 30 range. Thesenumbers are meaningless unless youhave a surface profilometer that canmeasure them (which a growing num-ber of performance shops now have).
CrosshatchCrosshatch is also important because theamount, depth and angle of the cross-hatch in the cylinder bores determineshow much lubrication the rings receiveand the rate of ring rotation.
Excessive shallow crosshatch anglescan hinder or slow down the necessaryring rotation that allows the rings to dis-sipate heat. It can also leave too much oilon the cylinder wall allowing the rings toskate over the surface and the engine touse oil. Too steep of a crosshatch anglemay not provide enough oil retentionand can result in dry starts and prematurering wear. A steep crosshatch angle canalso create excessive ring rotation thataccelerates ring and piston groove wear.
8 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009
ADVERTISERInformation Center
ADVERTISER PAGE # CIRCLE #
COMP PERFORMANCE GROUP 9, 17 109, 117
DIAMOND PISTONS COVER 3 121
DURA-BOND 2,13 102,113
ENGINE & PERFORMANCEWAREHOUSE
15, COVER 4 115, 122
HASTINGS 4, 7 104, 107
MAHLE CLEVITE 3, 5, 10, 11, 19 103,105, 110,111,119
MR. GASKET COVER 2,14 101,114
6-9 Bores/rings 4/15/09 10:53 AM Page 8
Ring manufacturers typically rec-ommend a crosshatch angle of 22° to32° from horizontal and uniform inboth directions.
Ring End GapsTradition said the end gaps on secondcompression rings could be tighterbecause the number two ring is notexposed to as much heat as the top ring.Today’s theory says it’s better to open upthe second ring gap 20 to 30 percent sopressure doesn’t build up between therings and cause the top ring to lose itsseal at high rpm. The result is bettercompression, better piston cooling andreduced oil consumption.
Any pressure that builds up betweenthe rings will blow down into thecrankcase,keeping oil out from betweenthe rings. This trick works best onengines that are running a dry oil sumpand pull a vacuum in the crankcase.
Some performance pistons also have“accumulator grooves” machined intothe piston land between the first andsecond ring grooves. The added spacetraps blowby gasses and helps preventthe top ring from unseating and flutter-ing.
For naturally aspirated engines, a topring end gap of .004˝ per inch of borediameter is often recommended for astock or moderate performance engine.For a 4-inch bore, that translates into atop ring end gap of .016˝to .018˝. Butthis will vary depending on the poweroutput of the engine.
For drag or oval track racing, the rec-ommended end gap is somewhat larger(.0045˝ per inch of bore diameter).Withfour-inch bores, that would be an endgap of .018˝ to .020˝.
For a nitrous oxide street perform-ance engine, the recommended end gapis .005˝ per inch of bore diameter (.020˝to .022˝ for an engine with four-inchbores). For a nitrous oxide drag engine,the recommended end gap for the topring is .007˝ per inch of bore diameter(.028˝ to .030˝ with four-inch bores).
With a turbocharged or superchargedracing engine, the top ring gap should be.006˝ per inch of bore diameter (.024˝ to
.026˝ with a four-inch bore).The recommended end gaps for sec-
ond compression rings are also the same,with slightly larger gaps if you want tominimize pressure buildup between therings.
The recommended ring end gap formost oil rings (except the new supernarrow one-piece rings) regardless ofengine application is typically .015˝.
Another trick to improve ring sealingat high rpm is to run pistons that havegas ports behind the top ring.Combustion pressure blows through theport to help seal the ring from behindand underneath. Some use vertical gasports with holes drilled from the top ofthe piston to the top ring groove justbehind the ring. Others use lateral gasports that are drilled through the bot-tom side of the top land and extend to
the back wall of the ring groove. Gasports work best at high rpm (above7,000 rpm) and are not recommendedfor street engines.
Getting rid of the end gap altogethercan also improve sealing, cooling andhorsepower. Gapless rings eliminate thegap between the ends of the ring byoverlapping slightly. Gapless rings areavailable in popular sizes with variouswear-resistant face and side coatings.Some engine builders who haveswitched to “gapless” top or secondcompression rings say they’ve gainedthree to five percent more horsepowerwith no other changes.Gapless rings aresaid to allow less than 1 cubic foot perminute (CFM) of blowby and on alco-hol-fueled engines, a gapless top ring orsecond ring helps keep alcohol out ofthe crankcase. PPMG
www.enginebuildermag.com | PERFORMANCE PARTS & MACHINING TECH GUIDE 9Circle 109 for more information
6-9 Bores/rings 4/15/09 10:53 AM Page 9
ngine cooling doesn’t onlyhappen at at the radiator.The valves (particularly theexhaust valves) take a lot of
heat from the combustion chamberand the valve seats have the responsibil-ity of helping to cool them off. Theseats draw heat away from the valvesand conduct it into the cylinder head,providing most of the cooling that thevalves receive.
Anything that interferes with theseat’s ability to cool the valves (such asa loose fit or deposits between the seatand its counterbore) can lead to prema-ture valve failure and expensive come-backs, so a cylinder head job oftenrequires valve guide and seat work torestore it for service or to improve per-formance. In order for a valve to seatcorrectly, for efficiency and power,engine builders must replace or bringback to spec all valve seats and guides.
When considering the steps neces-sary to repair or replace valve seats, beaware that with two types of cylinderheads you have three options: alu-minum with removable valve seats orcast iron, with removable or integralhardened seats. If the cast iron head hasintegral seats it will need to bemachined to replace the seat. If thehead is aluminum, the seat counterboremay have to be machined to accept anoversize seat if the bore is loose,deformed or damaged. Either way,you’ll need to figure the amount ofinterference that is required for thenew seat before cutting the head on aseat-and-guide machine.
Valve seat replacement is required ifthe cylinder head was warped andneeded to be straightened before resur-facing. Similarly, if an aluminum headwas cleaned by heating, the seats willneed to be replaced.
If the valve’s mating surface hasreceded below factory specifications orif machining the head would cause theseat to fall below factory specs, it mustbe replaced.
Replace the integral seats in a castiron head if the head has been groundbefore, because the hardened depth of
the head used in the seat area will betoo shallow to allow a second grinding.
If the valve seat insert shows evi-dence of being loose or doesn’t haveadequate interference; if there is evi-dence of corrosion on the cylinderhead around the outside diameter ofthe valve seat; or if there is evidencethat the seat has any cracking, burning,pitting or fissures, the seats must bereplaced.
The seat alloy and hardness mustalso be matched to the type of fuel usedand the engine application and com-patible with the type of valves that areinstalled in the engine.Again, there areoften differences of opinion regardingthe selection and use of various seatmaterials.
Nonintegral valve seats can fail for anumber of reasons. Most of the seatsthat end up being replaced are eithercracked or too worn to be reground orremachined. Seats can crack from ther-mal stress (engine overheating usually),thermal shock (a sudden and rapidchange in operating temperature) ormechanical stress (detonation, excessivevalve lash that results in severe pound-ing, etc.).
A small amount of valve recessionresults from normal high mileage wear,but it can also occur when unleadedgasoline or a “dry” fuel such as propaneor natural gas is used in an engine thatisn’t equipped with hard seats.Recession takes place when the seatsget hot and microscopic welds formbetween the valve face and seat.
Every time the valve opens, tinychunks of metal are torn away andblown out the exhaust. Over time, theseat is gradually eaten away and thevalve slowly sinks deeper and deeperinto the head.Eventually the lash in thevalvetrain closes up and prevents thevalve from seating.This causes the valveto overheat and burn. Compression islost and the engine is diagnosed as hav-ing a “bad valve.”The seat also has to bereplaced, but in many instances it maynot be recognized as the underlyingcause of the failure.
As a rule, most experts recommend
replacing OEM valve seats with onesthat are of a similar material – except incases where extra durability is requiredbecause of a change in fuels (convertingto propane or natural gas, for example),or an engine is being built for racing.
There are a number of differentvalve seat materials from which enginebuilders can choose. Many of thesematerials will work in a wide variety ofperformance applications while othersare designed primarily for special appli-cations such as industrial engines thatrun dry fuels like propane or naturalgas.The only consensus is that differentvalve seat materials can be used success-fully in most performance engines.
What kind of materials are we talk-ing about? Everything fromnodular/ductile iron alloys and powdermetal steel seats to hard aluminum-copper and bronze alloys, and berylli-um copper alloys. Many valve seat sup-pliers have their own proprietary alloyswhile others use industry standardalloys. But you don’t have to be a met-allurgist to appreciate the differencesbetween some of these materials.
A valve seat must do several things.It must support and seal the valve whenthe valve closes, it must cool the valve,and it must resist wear and recession.Consequently, a performance valve seatmaterial should provide a certainamount of dampening to help cushionthe valve when it closes at high rpm.Very hard materials, especially on theintake side, are not the best choice herebecause intake valves tend to be larger,heavier and close at faster rates thanexhaust valves.The wilder the cam pro-file, the more pounding the valve andseat undergo at high rpm.
Many late model domestic andimport engines have seats that are madeof powder metal. These types of seatsare very hard and durable, so they typ-ically show little wear at high mileages.Consequently, the seats may need littlework when the cylinder head is rebuilt.
One difference between cast alloyseats and powder metal seats is the waythe seats are manufactured. Cast alloyseats are made by melting and mixing
E
12 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009
12-15 Valve seats 4/15/09 10:58 AM Page 12
different metals together so they com-bine chemically. The molten soup isthen poured into a mold and cast toshape. The rate of cooling and subse-quent heat treatment of the metaldetermines its microstructure, hardness,strength and other physical properties.
Powder metal seats, by comparison,are made by mixing together variousdry metal powders such as iron, tung-sten carbide, molybdenum, chromium,vanadium, nickel, manganese, silicon,copper, etc.), pressing the mixed pow-ders into a die, then subjecting the dieto high heat and pressure (a processcalled “sintering”). This causes thepowders to bond together and form asolid composite matrix with very uni-form and consistent properties.
One of the advantages of powdermetal sintering is that materials that aredifficult or impossible to mix together
in a molten state can be blendedtogether and bonded to create totallyunique materials. For example, in pow-der metal bushings and ball joints,graphite is combined with steel tomake the material “self-lubricating.”
Another advantage of the powdermetal process is that parts can be man-ufactured very close to final tolerances,reducing the amount of machiningthat’s needed to finish the part to size.This lowers production costs andboosts manufacturing productivity.
The main reason why vehicle man-ufacturers have switched from cast alloyseats to powder metal seat inserts is toextend durability. Most late modelengines have to be emissions-certifiedto 150,000 miles or higher dependingon the application and model year. Ifthe valve seats can’t go the distanceduring durability testing, the vehicle
manufacturer can’t certify the engine.Powder metal seats are very good at
handling thermal stress as well asimpact stress, and typically show mini-mal wear after tens of thousands ofmiles of use.The homogeneous consis-tency of a powder metal seat alsoimproves heat transfer, which is goodfor the valves, too. Powder metal seatsalso tend to experience less micro-welding between the seat and valveeven at high combustion temperatures,which helps extend the life of bothcomponents.
Exhaust valves run much hotterthan intake valves so cooling is morecritical on the exhaust side. Heat trans-fer from the valve to the seat providescooling during the time when thevalve is closed, and by conduction upthrough the valve stem and into thevalve guide and head.
It’s more noticeable in performanceengines. Titanium valves do not shedheat as quickly as stainless steel valves,so the tradeoff for switching from steelto titanium to save weight is often hot-ter running valves.The higher the tem-perature of the exhaust valve, thegreater the risk of the valve causing apreignition or detonation problem.There is also increased risk of the valveburning.That’s why many suppliers oftitanium valves recommend seat mate-rials such as beryllium copper.
For racing applications using eitherstainless steel or titanium exhaustvalves, some suppliers recommend asintered valve seat insert, whichincludes a blend of finely dispersedtungsten carbide in a matrix of tem-pered M22 tool steel and special alloyiron particles.These powder metal seatshave a very uniform microstructure,and are highly machinable. Becausepowder metal seats work harden as theyage, they don’t have to be as hard ini-tially to provide good long term dura-bility, and the self-lubricating qualitiesof the material allows it to handle awide variety of fuels, including unlead-ed and leaded gasoline, straight alcohol,nitrous oxide and nitro methane.A shotof nitrous will cause combustion tem-peratures to soar, but the dose usually
14 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009Circle 114 for more information
12-15 Valve seats 4/15/09 10:58 AM Page 14
doesn’t last long enough to have anydetrimental effect on the seats.
The next step up is a high alloy seatmaterial, for applications where highheat resistance is required, such as apropane or natural gas fired stationaryengine but also for high performanceengines, heavy-duty and extreme dutyengines where longevity is a must. Seatsare made out of a high speed tungstencarbide tool steel,which gives it ceram-ic-like characteristics for extreme tem-perature resistance.
Conversely, because they tend torun much cooler than exhaust valves,low alloy seats work well with intakevalves in performance applications,even in such extreme cases as offshoreracing boats that run for hours on end.
Equipment SelectionSeats that are cracked, loose, sunken ordestroyed in some manner must bereplaced with new. Drilling, reaming,replacing valve guides, removing worn,loose or damaged valve seats, cuttingnew seat counterbores and machiningvalve seats are all part of the recondi-tioning process that your shop must beable to handle as efficiently as possibleand with precision and accuracy.
There are a few different kinds ofseat and guide machines available todayto fit most budgets. The first kind ofmachine is not much more complexthan a drill press and is often referred toas such. In these seat and guidemachines the tooling sets in a solid col-umn and the table floats to move fromguide-to-guide. These are very basicseat and guide machines that have somelimitations compared to moreadvanced machines, however, there arestill some small shops and hobbyiststhat use this style of machine. A smallshop may not be able to justify the larg-er,more expensive machines, and manyof these small shops simply don’t havethe volume to be concerned aboutdoing hand grinding and lapping.
The second level of seat and guidemachines is the floating powerhead sys-tem.These machines have become theindustry-standard for many enginebuilders today.This equipment, accord-
ing to the manufacurers we surveyed,run in the price range of about$18,000-$30,000, depending on theoptions and manufacturer you choose.On these machines, the cylinder head isstationary underneath a floating pow-erhead.The powerhead floats above onflat ways that adjust front-to-back andside-to-side.
The powerhead weighs much lessthan the cylinder heads, so when youcenter the powerhead, a proceduresimilar on most models, it floats andcenters with the pilot. Once it is cen-tered over the pilot, you let off the footpedal to lock it in place. This setupgives you a very rigid platform.
Modern cylinder heads often havecanted valves so on an older-stylemachine you have to tilt the fixture tomachine canted valve seats and guides.The fixture is relocated and set for eachguide.When using a modern seat andguide machine, the powerhead tilts,allowing you to adjust the powerheadfor whatever angle you need. Onceyou’ve set the powerhead angle youcan go up and down the line with thesame tilt angle for each valve seat. Somemachines can accommodate up to 15degrees of rotating tilt.
Today’s valve seat and guidemachines also use three-blade carbidecutters with three-angles or more.These cutters, or form tools, give you avery consistent profile and concentric-ity because the profile is built into thetooling. Manufacturers vary with thetype of tooling each offers, but all areessentially a form tool with built-inangles.
The third style seat and guidemachine is the “live pilot” design.Whereas a “dead pilot” design remainsstationary in the guide as the toolingrotates, a live pilot spins with the tool-ing in the guide.You have to be veryprecise when fitting live pilots intoguides otherwise too much play couldresult in a seat that isn’t completelyround. Some live pilot machines have asingle blade carbide cutter that canadjust while it spins to create almost aninfinite number of profiles. PPMG
www.enginebuildermag.com | PERFORMANCE PARTS & MACHINING TECH GUIDE 15Circle 15 for more information
12-15 Valve seats 4/15/09 10:58 AM Page 15
hich cam is right for you?You might as well askwhich color car is fastest.They’re both questions
that can only be answered with anemphatic “it depends.”
It depends on a variety of factors,each of which is determined by anumber of others.
Unless you are doing a totally stockrebuild and reusing the originalcamshaft, selecting a camshaft dependson what kind of engine you are build-ing and how that engine will be used.A stock engine for a daily driver isobviously an entirely different applica-tion than a big stroker motor for a ProStock racer. So how do you navigatethe daunting process of selecting the“best” camshaft for a particular engine?
One approach is to stick with whatworks. If you’ve used a particular camgrind before that delivers good torqueand horsepower for a certain kind ofapplication, you might want to play itsafe and stick with a tried-and-truegrind that has worked well in the past.But in today’s highly competitiveworld of professional racing, the hotcam, cylinder head and valvetraincombination that worked well last sea-son may not be the best choice for thisseason.
Since technology is constantlychanging with regard to cylinderheads, rotating assemblies, cylinderhead port configurations and engineblocks, intake manifold manufacturershave had to redesign many of theirmanifold plenums and runners to flowmore air for these stroker motors.Aftermarket engine blocks with largercylinder bores and bore spacing areadding more and more cubic inches ofdisplacement.
More and more camshaft profilesare required to fill the gaps not coveredby currently available cams.While thiscan make it much more difficult for anengine builder to pick an off-the-shelfcam that will deliver the best possibleperformance for a given combinationof engine parts, gearing and usage itmeans cam suppliers are more able
than ever to create custom camshaftsfor engine builders.
As long as an engine builder pro-vides technically accurate specifica-tions as well as detailed informationabout what he expects from theengine, many cam suppliers will makea custom cam for almost any engine.Sometimes the answer is an off theshelf cam. Other times the rightanswer is a custom cam.
The variables that must be consid-ered include all of the following:
• Engine Displacement – A smallerdisplacement engine usually needs ashorter duration camshaft for goodlow end torque and throttle response.A large displacement stroker motor, onthe other hand, can usually handlemore cam duration without sacrificinglow end torque and throttle response.
• Bore & Stroke – Long strokeengines develop more torque thanshort stroke motors, but short strokemotors can typically rev much higher.So for a high revving engine, you wanta cam that develops power from maybe3,500 rpm up to 9,500 rpm. A bigstroker motor, on the other hand, maynever see the high side of 6,000 rpm,so it would need a cam that works bet-ter at lower rpms.
• Rod Length – Rod length affectsthe angularity and torque on thecrankshaft as well as piston speed, fac-tors that are used to optimize airflowin the desired rpm range.
• Engine RPM Range – Are youbuilding a low rpm torquer motor or ahigh revving engine? The cam grinderhas to know where you want maxi-mum torque to occur so he can choosethe optimum duration, lift and lobeseparation numbers for your cam.
• Compression Ratio – The higherthe static compression ratio, the moreduration the engine can handle with-out going into detonation.
• Cylinder Heads – Are the headsstock, modified stock or aftermarket?If aftermarket, which brand and modelof aftermarket head? Are they off-the-shelf heads or have the ports beenreworked or CNC machined? If so,
what are the port runner volumes?How big are the intake and exhaustvalves? How about the volume of thecombustion chambers (open chamberor small chamber)? Are the heads alu-minum or cast iron? Aluminum runscooler and can handle more heat withless risk of detonation.All of these fac-tors influence the lift, duration andoverlap that will be ground into thenew cam profile.
• Valvetrain – What is the ratio ofthe rocker arms? Are they stock orhigh-lift, and if so what is their exactratio? This will affect both cam lift andduration at the valves. Are the rockerssteel or aluminum? How stiff are thevalve springs? Can the springs andpushrods handle the anticipated rpms?The cam has to work with the lifters,pushrods, rocker arms and valvesprings to achieve the desired lift, dura-tion and rpm you want to achieve.
• What Type of Cam and Lifters? Ifyou want a flat tappet cam, will you beusing oversize lifters? Roller cams costmore but provide a significant reduc-tion in internal engine friction, andtypically make more torque and horse-power because a roller lifter can handlea much steeper ramp on a cam lobethan a flat bottom lifter. This allowsroller cams to open the valves morequickly so they can reach maximumlift earlier in the timing cycle. If youplot valve lift and duration on a graph,the area under the curve of a rollercam with an aggressive lobe profilewill be greater than that of a flat liftercam for the same lift and durationspecifications.This makes more power.
• Induction System – Will theengine be naturally aspirated, boostedwith a supercharger or turbocharger (ifso, how much boost pressure?), and/orfitted with a power adder (nitrousoxide)? Blown engines typically runbetter with slightly milder cams thathave a wider lobe separation to reducevalve overlap. Does it have a carburetoror more than one carburetor? If so,what’s the cfm rating of the carbure-tor(s). If the engine is fuel injected,what type is it (multiport or throttle
W
16 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009
16-20 Cam bores/bearings 4/15/09 11:00 AM Page 16
body, original equipment or aftermar-ket)? How big is the throttle body?What kind of manifold is on theengine (brand, split-plane or single-plane, low rise, high rise, tunnel ram ormulti-carburetor)? Split-plane mani-folds typically produce more low endtorque and are better for streetengines.
• Firing Order – Is it stock, or areyou swapping the firing order oncylinders #4 and #7 on a big block orsmall block Chevy for a broadertorque curve? Obviously, you’ll need adifferent cam if the firing order hasbeen changed.
• Street, Street/Strip or Race Only– If the engine is being built for thestreet, will it be emissions exempt orwill it have to meet emission regula-tions? If a customer wants you to buildhim or her a street/strip engine, howmuch time will it actually be run onthe strip? The customer may believe itwill be a 50/50 split, but realistically itwill likely be more like 95 percentstreet and 5 percent strip. That affectswhere you want the engine to developpeak torque for everyday drivability.
• Transmission – Manual orAutomatic? Automatics typically needmore low end torque, especially in aheavier vehicle. But this can be affect-ed by the stall speed of the converter.A higher stall speed allows the engineto rev higher before it grabs, but hurtsfuel economy in a street driven vehi-cle. If the engine will go in front of amanual transmission, how many gearsdoes the transmission have, and is thegear spacing wide ratio or close ratio?
• Final Gearing – The final driveratio of the differential and the size ofthe tires will also affect the rpm of theengine as it accelerates and cruises.Theengine’s power curve should match thegearing for optimum performance.
• Exhaust System – If the enginehas headers, what is the style, diameterand length of the headers? Does theexhaust system have mufflers or across-over H-pipe or X-pipe?
• What Does Your Customer Want?Does the customer you are buildingthe engine for want lots of low end
torque or high rpm power? Does hewant a lumpy idle or a smooth idle? Ishe willing to spend more for a customcam or will an off-the-shelf cam begood enough?
You can often select cams onlineusing software programs that literallywalk you through the process.However one of the most commonmistakes novice engine builders andDIYers alike make is over-camming anengine.
They want the cam with the biggestnumbers, never mind the fact that sucha cam may be a poor match for theengine or vehicle they are sticking itinto.A wild cam in a slightly modifiedengine may sound great with a lopingerratic idle. But if the engine has nolow end torque or throttle responseand never reaches the rpm rangewhere the cam starts to really work,the engine will be a dog.
Understanding DurationDuration is one of the factors in camdesign that affects where the camdevelops peak power. Duration is howlong the cam lobe holds a valve openand is specified in degrees of crankshaftrotation. The duration specificationwill vary depending upon how andwhere it is measured.
As the cam spins around and a lobebegins to push its lifter up, the valvestarts to open. But if the engine hassolid lifters, the lash in the valvetrainmust first be compressed before thevalve starts to open. If duration ismeasured from the point where thelifter has risen .004˝above the base cir-cle of the cam lobe to when it comesback down to within .004˝ of base cir-cle, it creates a somewhat inflatedvalue. Some cam manufacturers referto this as the “advertised duration”because it gives the biggest numbers,and thus appeal to engine builderswho subscribe to the “bigger is alwaysbetter” philosophy of cam selection.
By comparison, the Society ofAutomotive Engineers (SAE) methodfor measuring duration says it is to bemeasured at .006˝ above the base circlefor hydraulic cams, and .006˝ plus the
specified valve lash for mechanicalsolid lifter cams.
Another way duration may be spec-ified is to measure it at .050˝ above thebase circle of the cam lobe. The.050˝specs are the ones most com-monly cited in aftermarket catalogs.
What does a duration spec tell youabout a cam? It reveals somethingabout the rpm range where the camwill make the most power. Generallyspeaking, the longer the duration thehigher the rpm where the cam makespower. Short duration cams are goodfor low speed torque and throttleresponse while long duration camshold the valves open longer for betterhigh speed breathing and top endpower.
Cams with durations in the 195 to210 degree range (measured at.050˝cam lift) are usually consideredbest for stock unmodified engines andthose with computerized engine con-trols. Once you go beyond 210 to 220degrees of duration, intake vacuumstarts to drop. This upsets idle qualityand affects the operation of computer-ized engine control systems.
Performance cams typically havedurations ranging from 220 up to 280degrees or more. The longer the dura-tion, the choppier the idle and thehigher the cam’s power range on therpm scale. A cam with a duration of240 degrees or higher will typicallyproduce the most power from 3,500rpm to 7,000 rpm.
If you try to compare durationspecs between different cams, though,you don’t always get an accurate com-parison because duration specs don’ttell you anything about the lobesthemselves. Though cams from twodifferent manufacturers may haveidentical lift and duration specs, thelobes on one cam may be ground dif-ferently from those on the other. Onecam may have more of a peak shapedlobe while the other has a “fatter” lobe.A “V” shaped lobe will breathe differ-ently from a “U” shaped lobe becauseit doesn’t hold the valve at its maxi-mum opening as long.
One lobe profile may also close the
18 PERFORMANCE PARTS & MACHINING TECH GUIDE | April 2009
16-20 Cam bores/bearings 4/15/09 11:00 AM Page 18
valve more softly than the other, to reduce valve bounce athigh speed.Valve float can also be a problem with lobes thatchange shape abruptly unless valve spring pressure isincreased.The profile of the lobes on one cam may also bethe same on both the up and down sides of the lobe (whichis the norm for most stock and street performance cams)compared to an “asymmetrical” grind (different profiles oneach side of the lobe) on the other cam.
The only way to really compare cam grinds, therefore, isto measure and plot lift versus rotation on a graph.This canbe done manually with a degree wheel and dial indicator(which is a tedious job), or with an electronic stylus thatplots the results on a computer.
Overlap and SeparationAnother spec you need to look at when selecting a cam isthe relative timing of the intake and exhaust valves.This canbe expressed either as “valve overlap” (the time duringwhich both the intake and exhaust valves are both open) or“lobe separation” (the number of degrees or angle betweenthe centerlines of the intake and exhaust lobes). Decreasingthe lobe separation increases overlap, while increasing theseparation decreases overlap.
Most stock replacement cams with durations of less than200 degrees will have lobe separations of 112 to 114degrees. Higher duration cams for mid-range performancetypically have 110 to 112 degrees of lobe separation.Withracing cams, you’ll find lobe separations that range from 106to 108 degrees for more valve overlap.
Overlap occurs when the intake valve starts to openbefore the exhaust valve has finished closing. Increasingoverlap can be a desirable thing in a naturally aspirated highrpm engine because the outgoing exhaust actually helpsscavenge the cylinder to draw more air and fuel into thecombustion chamber. But too much overlap at low rpmkills low end torque and throttle response by reducingintake vacuum excessively. It can also create idle emissionproblems by allowing unburned fuel to be drawn throughinto the exhaust.
Why Bore?There are three basic reasons for line boring the main bear-ing and cam bearing bores in engine blocks. One is torestore worn, out-of-round or damaged bores. If an engineoverheats or loses oil pressure, one or more bearings on thecrankshaft or camshaft may seize and spin. The resultingdamage to the bearing bore must then be repaired by eithermachining the hole to accept a standard sized bearing or anoversized bearing.
With main bearings, a worn, out-of-round or damagedbore can be restored back to standard ID by grinding ormilling the mounting surface of the main caps, bolting thecaps back on the block, and then cutting the holes back totheir original dimensions.
In the case of worn, out-of-round or damaged cam bear-
ings in an engine block, there are no removable caps.Theonly option is to enlarge the bores so new oversize cambearings with a larger outside diameter (OD) can beinstalled.
Reason number two for line boring a block is to restoreproper bore alignment - a process which is often called“align” boring (or honing if a line hone is used instead of aboring bar). As rigid as an engine block might seem, thereis actually quite a bit of residual stress in most castings.As anew “green” block ages and undergoes repeated thermalcycles, the residual stresses left over from the original cast-ing process tend to distort and warp the engine.This affectsthe alignment of the crankshaft and camshaft bores as wellas cylinders.
Eventually things settle down and the block becomesmore or less stable (a “seasoned” block).The bearings as wellas the crankshaft and camshaft journals gradually developwear patterns that compensate for the distortion that hastaken place.
Additional warpage can occur if the engine is subjectedto extreme stress (like racing) or overheats. If the originalcrankshaft or camshaft is then replaced without align bor-
www.enginebuildermag.com | PERFORMANCE PARTS & MACHINING TECH GUIDE 19Circle 119 for more information
16-20 Cam bores/bearings 4/15/09 11:00 AM Page 19
ing the block, it may bind or causerapid bearing wear. Likewise, if you’rebuilding a high performance enginewith close tolerances, you don’t wantany misalignment in the main bores orcam bores.
The third reason for line boring orhoning a block is to correct or changebore centers or bore alignment (aswhen “blueprinting” a high perform-ance engine).The camshaft and crank-shaft should be parallel in the block. Ifthey are not, line boring can correctthe misalignment to restore the propergeometry. With performance engines,there may also be a reason to changethe centerline of the crankshaft orcamshaft slightly to alter the piston orvalvetrain geometry.
Even though the end goal of lineboring has remained constant, someinnovations in the recent past havemade line boring anything but a bor-ing subject.
One of the disadvantages of using atraditional horizontal boring bar is thatit tends to sag.This has to be counteredby using adequate support so all thebore holes are cut straight and truewith no misalignment between holesand no variations in bore size.
One way to eliminate the effects ofgravity on the boring bar is to use avertical boring machine. Rotating theblock and bar 90° so the block and barare straight up and down provides atruer, straighter cut says one manufac-turer of this type of equipment. It alsosaves floor space because the machinehas a smaller footprint.
Another way to circumvent theissue of bar sag is to use a 90° rightangle cutter attachment on a millingmachine. Instead of using a long steelbar to pass single or multiple cuttersthrough the main bores, the 90° cutteris lowered into the space between eachmain bore, then moved sideways tomachine the bore ID. It’s sort of likeworking around a corner.
With computer numeric controls(CNC), each hole can be preciselymachined to exact dimensions and thecenterline of each hole perfectly locat-ed and aligned with all the rest. This
technique works especially well onlarge, heavy blocks that may be toolong for most boring bars.
OHC ApplicationsWhen overhead cam cylinder headscame into widespread use, it quicklybecame apparent that line boring orhoning would be needed to correct avariety of problems (cam bore wear,distortion and damage as well as cylin-der head warpage). Aluminum headscan be easily warped by overheating.When the head gets hot, it tends toswell the most in the center area.Thehead bulges up in the middle causingmisalignment in the cam bores.This, inturn, can cause uneven wear in thecam bores, camshaft sticking or evencam breakage.
If an OHC cam won’t turn in thehead, it means the cam is stickingbecause either the cam is bent or thehead is warped. In the case of a bentcam, the center cam bores will showexcessive wear or be worn out ofround. If the head is warped (whichmany aluminum heads are), the headshould be straightened BEFORE it isline bored or resurfaced.
In the past, a popular fix for OHCheads with worn cam bores was to linebore the head to accept bearing shellsor inserts. Nowadays, the more popularfix seems to be cutting the head toaccept a cam with oversized journals.
Alignment ChecksThe alignment of the crankshaft andcamshaft bores in blocks and OHCheads can be checked by placing astraight edge in the bores or along thebore parting lines and using a feelergauge to check for misalignment.
How much misalignment is toomuch? It depends on the engine andthe application. A light duty passengercar engine is not as critical as a highrevving performance engine or a hard-working diesel engine. As a rule, mostpassenger car and light truck enginescall for .002˝ or less of misalignmentbetween all the bores, and .001˝ or lessmisalignment between adjacent mainbores. For performance engines, you
can reduce these maximum tolerancesby half or more.
Another dimension to look at isbore wear. Bore diameters should usu-ally be within .001˝ of specifications tosupport the bearings properly, with nomore than .001˝ out-of-roundness ifthe horizontal dimension is greaterthan the vertical dimension.
Also check for wear on the thrustbearing surface of the main cap. Ifworn or damaged, this surface shouldalso be remachined.
Installation TipsFor stock, street performance andsome racing applications, a cam kit thatincludes lifters is a good way to go.Some kits also include new, stiffer valvesprings for higher revving applications.But for professional racing, most camsare sold outright.This gives the enginebuilder more flexibility in choosinglifter configurations.
Regardless of what kind of engineyou are building, or the application it isgoing into, always use new lifters witha new camshaft. Reusing old, wornlifters can ruin a new cam.
Be sure to check valve-to-pistonclearance, especially with high lift camsand/or high lift rocker arms. Also,make sure the springs do not bind withhigh lift cams or rockers.
Lubrication is also critical. All camlobes should be coated with a highpressure assembly lube, not just motoroil (which will run off if the enginesits for any period of time).Also, if theengine has a flat tappet cam and willbe used for racing, special racing oilthat contains higher levels of zinc thanordinary motor oil may be necessaryto prevent premature lobe and lifterwear.
Camshaft break-in after the initialengine start up typically requires run-ning the engine at 2,000 to 2,500 rpmfor up to 30 minutes. Don’t let it idleor you may wipe out a lobe. Be sure tocheck valve-to-piston clearance, espe-cially with high lift cams and/or highlift rocker arms. Also, make sure thesprings do not bind with high lift camsor rockers. PPMG
20 MACHINING & ENGINE PARTS GUIDE | April 2009
16-20 Cam bores/bearings 4/15/09 11:00 AM Page 20
