USAC.25Briggs&StrattonWorldFormulaTechManual 1

BRIGGS&STRATTONWORLDFORMULA–TechManualUpdatedNovember1,2012

FROM2013USACNATIONAL.25MIDGETRULEBOOK,APPENDIXI731EngineProtestRules(appliestoHondaandBriggsclassesonly)

1. Protestshallbefromwithinthesamedivisionofclassonly,i.e.Jr.,Sr.,Lt.&Hvy.120‐

160,AnimalorWorldFormulaonly.Competitorsinthesamedivision,andinthesameracemaymakeaprotestonanengine.NoprotestinginRookieClass.Handlersmaynotprotestmorethanonecarpereventandmaynotprotestsamedrivermorethanoncepercalendaryear.

2. HondaEnginesandWorldFormula/AnimalEnginesmaybeprotestedfor$400.00cashonlyplusanyapplicableshippingchargesifnecessary.Noprotestedrelatedinspectionwillbestartedpriortothefundsbeingpostedwiththeproperofficial.

3. ThisprotestformandcashmustbesubmittedtotheChiefSteward,orhis/herdesignee,beforetheendoftheracethattheprotestedengineisparticipatinginI.E.Checkeredflaglapcomplete.

4. TheprotestcanonlybemadeduringanA‐Mainevent.5. Thepersonprotestingthemotormusthavetheirengineinspectedforcompliancefirst.

Ifthe“protester’s”engineisfoundillegaltheprotestisnullandvoidandtheprotestfeewillgototheclub.Ifthe“protester’s”engineisfoundlegaltheprotestwillcontinue.

6. TheChiefSteward,his/herdesignee,willholdtheprotestmoneyuntiltheprotestedenginehasbeeninspectedforlegality.Theprotestedengineshallbetagged/markedandsealedassoonasitcarcomesacrossthescaleifithasnotbeensealedprior.

7. Theprotestedengineaswellastheengineoftheprotestedpartyshallbeimmediatelytakentoimpoundand/orpresentedtotheTechDirectorforinspection.Enginemustremaininimpoundandinthepossessionoftechofficialsthroughouttheentireprocess,includingshippingtoUSACHeadquartersordesignatedtechinspectionstationandthetransferringoffunds.

8. Bothprotesterandprotesteehavetheoptiontobepresentatthetimeofinspection.9. Anyprotestthatiswithdrawnwillbeassesseda$50.00feethatwillbepaidtothehost

club.10. Iftheprotestedengineisfoundtobeillegal,themotormustbecompletelytorndown

tocheckforadditionalillegalities.TheTechDirectormustconfiscateallillegalpartsandrelatedpartsfromtheprotestedengineandshallimmediatelyforwardthemtotheUSACHeadquarters.Ifengineisfoundillegalprotestmoneyminus$50plusanyshippingcostwillbereturnedtothepersonfilingtheprotest.

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11. Refusalofprotest,destroyingorwithholdingofpartsoranyotherlackofcooperationinthisprotestorinspectionprocessshallbeinterpretedasanadmissionthattheengineisillegalandshallsubjectthedriverandhandlertotheconditionssetforthintheSuspensionsProgram.

12. Anytechedorprotestedengine,blockorpartwhicharedeemedtobeovermaximumwearlimitsinoneormorespotsbutisundermaximumwearlimitsinotherspotsissubjecttoconfiscationbutnotDQ’able.

13. Note:ReferencetoConfiscationduetoWearLimitsin“EngineBlockInternalRules”ofbothManuals.

14. Iftheengineisfoundlegal$400willbegiventothepersonwhoseenginewasprotested.

732EngineSuspensionRulesHandlersanddriversguiltyofhavinganenginedeclaredillegalattechnicalinspectionsshallbedisciplinedasfollows:

1. Firstoffense–upto30‐daysand/or4racesuspensionforhandleranddriverfromparticipatingintherespectiveclassatanyUSACSanctionedevent.

2. Secondoffensewithinoneyearoffirstinfraction–One‐yearsuspensionforhandleranddriverfromparticipatingintherespectiveclassatanyUSACSanctionedevent.

3. Thirdoffensewithintwoyearsoflastinfraction–SuspendedforlifefromUSAC’s.25Midgetdivision.

4. SuspensionforlifeisopentoreviewbyUSAC.5. Suspensionshallbeginimmediately.6. IllegalHonda,AnimalandWorldFormulapart/sshallbesentwithinfiveBusinessdays

totheUSACofficeordesigneeforreview.TheTechdirectorhas48hourstodetermineifthepart/sarelegalorillegal.Ifthepart/saredeterminedtobelegalitshallbereturnedtohandler.Handlershallbenotifiedifpart/sarelegalorillegal.Allillegalorconfiscatedpart/sshallbesenttoNationalTechDirector.Alllegalpartsshallbereturnedtohandler.

7. IfaHondamotorisfoundtohaveavalveoilsealduringtechitshallbearacedisqualificationonly.

8. Sparkplugsandexhaustinfractionsarearacedisqualificationonly.9. Failuretogototechand/orimpoundwillresultinaracedayDQ.

Refusaloftechshallbeinterpretedasanadmissionthattheengineisillegalandasuspensionfromtheclassshallbeimmediatewithallawardsandqualificationsbeingrevokedwithasix‐monthsuspensiondriverandhandlersuspensionatanyUSACSanctionedevent.

10. Forthepurposeofthisruleonly,ifahandlerhasmultiplecarscompetingatoneraceeventandmorethanoneengineisfoundtobeillegalatthatevent;itwillbeconsideredtobeoneoffense.

11. AllmembershipsuspensionsmustbesenttotheNationalTechDirectorwithin5BusinessDays.

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12. IllegalRookieenginepartsshallbeconfiscated(HondaorAnimal)butthesuspensionshallnotbeleviedagainsthandlersordriversforthefirstoffense.Thesecondoffenseshallresultina30daysSuspensionfromRookie.

13. Thecosttoappealasuspensionis$175plusanyassociatedfees.Theappealmustbemadewithin3daysoftheruling.

UNLESSITSAYSYOUCANDOITYOUCANNOTDOIT!!!

GENERALRULES

1. OnlystockBriggs&StrattonWorldFormulaModel#124435‐8101willbeusedinthisclassexceptasprovidedinthisTechmanual.AllpartswillbestockunalteredBriggs&StrattonWorldFormulapartsspecificallymadeforthisenginebyBriggsandStratton.

2. DirectDrive:Clutchesarenotallowed.3. UnlessotherwisespecificallyrequiredorallowedbythisTechmanualnomachiningor

alterationsofanykindwillbeallowedtotheWorldFormulaengineorreplacementpartstobeusedinanyWorldFormulaengineforUSACunlessspecificallystatedintheserules.ALLPARTSARESUBJECTTOCOMPARISONWITHAKNOWNSTOCKPART

4. Modificationsormachiningofanypartsinordertobringthemtostatedminimum/maximumspecs.,(orforanyreason).“Blueprinting”isnotlegal.

5. Fuel:Gasolineonly,noadditives.TechprocedureisperUSACRuleBook.6. ALLPERTINANTPENELTIESWILLAPPLY.

REQUIREDMODIFICATIONS

1. Gearbox:Anygearboxisapproved.Crankshaftmaybealteredonlyonexternaloutputshaftbyaddingageartodrivethegearbox.Allothermodificationstocrankshaftareillegal.

2. ElectricStarter:Electricstartermustberemovedandblowerhousingopeningsmustbecoveredwithafabricatedmetalcover.Youshouldnotbeabletoseeanyoftheflywheel.

3. RecoilStarter:Recoilstartermustberemoved.Startercupmustbecutdownorreplacedwithaflatwasher,BriggsPartnumber691736forflatwasher.

4. Exhaust:Mustuseexhaustportextensioninport.Minimumofa0.520lengthmeasuredfromthebottomoftheexhaustflangetotheendofthepipeusingadepthmic.Anystylepipeandmaxoffour(40B&Smufflersonly).

5. OilBreather:Oilbreathermustventtocatchcanunderthetailconewithintheenginecompartment.

6. CarburetorOverflow:Carburetoroverflowmaybeventedtotheground.7. ImpulseFittingonIntakeManifold:Impulsefittingonintakemanifoldmustbefiledif

impulsetypefuelpumpisnotused.

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ALLOWABLEMODIFICATIONS1. CrankshaftSeal:Notechonthecrankshaftsealongearboxside.2. FlywheelCup:Flywheelcupmaybecutorreplacedwithawasher,BriggsPartnumber

691736.3. FuelPump:Fuelpump,B&Spartnumber808656maybeused.Ifused,fuelpumpmust

bepulsedfromtheintakemanifoldpulsefittingonly.4. BlackPlasticControlCover:BlackcontrolcoverB&Spartnumber557048andbracket

#555618includingON/OFFswitchmayberemoved.Maybereplacedwithametalcoverplate(BriggsPlatePartnumber555699)andboltedtotopofengine.Blowerhousingopeningsmustbecoveredwithfabricatedmetalcover.

5. RockerCover:RockercoverB&Spartnumber555679or499924maybemodifiedforinstallationofoilbreatherlinefitting.(itisallowabletohavesecondarydrillingofholesinthebreathervalveareaofthevalvecover.)

6. TheinstallationoftheBriggsBreatherBy‐passsystem,partnumber555688,perincludedms‐3742sheet.Mustbeteeintocatchcan.

7. PlasticFlywheelRingGear:Plasticflywheelringgearmayberemoved.Nootheralterationsincludingtheadditionorsubtractionofmetaltotheflywheelallowed.

8. HeatDispenser,Briggspartnumber555690,maybeinstalledperincludedms‐3758.9. CanuseBriggsPVLsystem:PartNumber557127.10. Manifoldadaptor:PartNumber55769.11. HeadGasketFireRing:PartNumber555698.

TECHPROCEDURE

1. Generala. Heli‐Coiledthreadsforshrouds,(all),valvecover,oildrain,oilfillholes,blower

housing,andexhaustpipeattachstudsontheheadandlowerbrackets.b. Blockingairflow:Nodevicemaybeusedthatwill,orappearsthatitmayimpede

airflowintotheenginecoolingsystem.Thismayrequirethattheengineberunataspeedaboveidlebythetechpersonnelatthescale,afterthecarhasqualifiedorraced.

CARBURETOR

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RemoveCarburetor:

1. StockWalbroPZ26carburetorendingincalibrationnumber.A63Carbonly.Noalterationsallowed.Slidemustremainunaltered.Stockneedlemarked“CDB”isrequired.

2. NeedleJetC‐clipmustbeproperlyinstalledbutmaybeinstalledatanyofthe5factorysettingsontheneedlejet.

3. Throttlecablecaponthetopofthecarburetormustbeusedandproperlyinstalled.4. Choke:Unaltered,butmaybefastenedopenwithaspring,rubberband,orziptie,or

removechokeandplugwithsilicone.5. Idlejetsanysizeallowed.6. Mainjetsanysizeallowed.7. VenturiMeasurement:

Vertical:25.00mm=/‐.15mm.Horizontal:18.50mm=/‐.25mm,00mm

8. CarburetorAdapter:CarbadapterB&Spartnumber557050orupdatedB&Spartnumbermustbeusedinstockconfiguration.

9. Airfilter:B&Spartnumber698973istheonlyairfilterallowedbutisnotrequiredtobeused.

10. Carburetoroverflow:maybeventedtotheground.11. O‐RingpartnumberB&Spartnumber557007isrequiredandmustbeunaltered.

WorldFormulaIntakeManifold CarburetorAdapter ENGINECOOLINGSHROUDS/BLOWERHOUSING

1. Allpiecesofthestockenginecoolingshroud/blowerhousingmustbestockandproperlyinstalled.

2. Startercupmaybecutdown.3. Removeblowerhousing.4. RemoveValvecover.

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Checkvalvelift&ignitiontiming:1. Max.valveliftwillbecheckedfromthetopofthevalvespringretainer.Valvesmustbe

adjustedtozeroclearance.2. ValveLift:firstcamshaftcheckwillbetakenatthevalvespringretainers.Withthelash

setatzero,themovementofthevalvespringretainersmaynotexceedthefollowing:Intakeandexhaust:.3085inchesmax.

3. Installdegreewheel,usingpositivestopmethod.Checkignitiontiming.Withtherightedgeofthemagnet,(notthemagnetholder),alignedwiththerightedgeofthenotchoftherightlegofthecoil,theenginemustbefrom23degreesBTDCto27degreesBTDCyoumustusestockkeywayandtheenginemustbefrom23degreesBTDCto27degreesBTDCyoumustusestockkeywayandunalteredflywheelstockBriggsandStrattononlyallowed.

4. Techcamshaftatpushrods.Pushgentlydownondialindicatorstemtoensurethatthereisnolashwhenpushrodsaregoingdown.

CYLINDERHEAD&HEADGASKET

1. Removecylinderhead.2. HeadGasket:

a. Stock,unalteredB&Spartnumber555698fireringgasketallowed.b. Minimumgasketthicknessbetweenheadboldholes.041inches.Measurements

aretobemadewithdialcaliperfrominsideoffirering.

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3. Head:a. Cylinderhead,557101or557133(includestheheatdispenserinstalled)mustbe

stock,unalteredandbein“ascast”andinfactorymachinedcondition.Noadditionstoorsubtractionsfromanypartofheadwiththeexceptionoftheheadgasketsurfaceandtheheatdispenser.Briggspartnumber555690maybeinstalledperincluded.Cylinderheadgasketsurfacemaybemachined.Depthfromgasketsurfacetotheheadsurfacebetweenvalvesmustbeaminimumof.319.Hardcarbonmayberemoved.

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b. Exhaustpipeattachmentstudmaybeheli‐coiled.c. Noalterationsofanykindmaybemadetotheintakeorexhaustports.d. Combustionchamber:19.6ccto20.6cce. IntakePort:

Maximumdiagonalmeasurement=1.101inches.Maximumverticalmeasurement=1.044inches.

f. ExhaustPortASCAST:MaximumIDofshoulderinbottomofexhaustport=.854inches.

g. ValveSeats,Intakeandexhaust:Mustremainfactoryspec.withone45degreeangleonly.Multianglevalveseatsarenotpermitted.

h. Intakevalveseatdiameterinside=.965to.972inches.i. Exhaustvalveseatdiameterinside=.844to.850inches.

RemoveValvesInspectretainersforalterationsthatwouldincreasevalvespringpressure.BothintakeandexhaustmusthavestockB&Svalvekeepers.VALVES

ExhaustValveAssemblyIntakeValveAssembly

1. Checkvalvesfordimensionsandweight.StockandunalteredB&Spart#557018(exhaust)and#557017(intake).Valvesurfacemustbeunalteredfactorygroundandhaveone45degreesealingsurfaceonly.Therewillbenootheranglesgroundonanypartofthevalvesurfaceonly.Therewillbenootheranglesgroundonanypartofthevalve.

2. Lappingofvalvesallowed.3. ValveGuides:ReplacementofvalveguideswithB&Sfactorypart#555645onlyis

allowed.

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4. IntakeValve:

MinimumWeightofValve 27.90gramsmin.Diameterofvalvestem .246to.247Diameterofvalvehead 1.055to1.065inchesDiameterofvalveseat .965to972inchesIDValvelength 3.372+/‐0.010

5. Exhaustvalve:Minimumweightofvalve 27.70gramsmin.Diameterofvalvestem .246to.247Diameterofvalvehead .935to.945Diameterofvalveseat .844to.850inchesID

Valvelengh 3.372+/‐0.010VALVESPRINGS

1. ValveSpringswillbedualcoilstock,unalteredB&Spart#557024. 2. InnerSpringinnerWireDiameter .066to.068inches 3. OuterSpringOuterWireDiameter .112to.114inches

ROCKERARMS,ROCKERBALL,ANDROCKERARMSTUDS

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1. RockerarmswillbestockB&Spart#555711or#797443andwillnotbealteredinanyway.

2. Rockerstudswillbestock,unalteredB&Spart#699230or#797441andinstocklocation.

3. RockerBall#694543or#797440mustbestock.Diameter.590inchmin.to.610inchmax.holes.Nobendingofstuds.Rockerarmstudplate#698214or#797442mustbeboltedtotheheadwithonestockB&Sgasketonly‐noalterations.

4. *OptionalUpgrade*5. RockerarmswillbestockB&Spart#555546or#797443andwillnotbealteredinany

way.6. Rockerarmlength2.865inchesMinimum.7. Rockerstudswillbestock,unalteredB&Spart#555549or#797441ANDinstock

location.#797443and797441arepartofrockerarmadjusterkit#797440oldandnewstylepartsmaynotbemixed.

8. RockerBallmustbestock.9. Diameter0.590inchmin.to0.610inchmax.

PUSHRODS

1. Pushrodswillbestock,unalteredB&Spart#693517. ENGINEBLOCK

1. Engineblockmustbein“ascast‐stockfactorymachinedconditionwithnoalterations.Theremustbenoadditionorsubtractionsofmetaloranysubstancetotheinsideoroutsideofthecylinderblockexceptforcylinderblockdeck.

2. Machiningofdecksurfaceispermitted.Hardcarbonmaybescrapedfromthepiston.Pistonpopupcannotexceed0.038aboveblocksurfaceinthecenterofthepiston.Whenmeasuringpistonpopup.Itshouldbeaccomplishedwithbarstockonaparallelwiththepistonwristpin.

USAC.25Briggs&StrattonWorldFormulaTechManual 11

1. Cylinderborewillnotbeboredoversize.2. Cylinderborewillnotbere‐sleeved.3. Cylinderborepositionwillnotbemovedortippedinanymanner.4. Cylinderboredimension:2.688inches+/‐.005max.takenattoporbottomofbore.5. Checkstroke;2.204max,pushpistondowntotakeuprodplay.

CAMSHAFTPROFILELIMITS

1. Thefirstcamshaftcheckwillbetakenatthevalvespringretainers.Withthelashsetatzero,(0),themovementofthevalvespringretainermaynotexceed.3085inches.Camshaftmustbesuppliedwithcompressionrelief.

2. Techcamshaftatpushrods.Pushgentlydownondialindicatorstemtoensurethatthereisnolashwhenpushrodsaregoingdown.

INTAKELOBE LIFT EXHAUSTLOBE13TO17BTDC .050 230to234BTDC0to4BTDC .100 214to218BTDC14to18ATDC .150 200to204BTDC30to34ATDC .200 184to188BTDC50to54ATDC .250 163to167BTDC64to65ATDC .275 148to152.5BTDC303Min.309Max. 303Min.309Max.(107to111CL.) (108to110CL)149to153ATDC .275 65to72BTDC163to167ATDC .250 52to56BTDC183to187ATDC .200 32to36BTDC200to204ATDC .150 15to19BTDC

USAC.25Briggs&StrattonWorldFormulaTechManual 12

214to218ATDC .100 4to0BTDC230to234ATDC .050 13to17ATDCFLYWHEEL

1. Nomodificationsallowedtoflywheel.2. StockB&Spart#557126only.Nomachining,glassbeading,sandblasting,paintingor

coatingofflywheelisallowed.3. Flywheelswithbrokenfinsmustbereplaced.4. Plasticstarterringgearbetweenflywheelandenginemayberemoved.5. Stock,unalteredflywheelkeyisrequired.Nooffsetkeywaysallowed.6. Canuseanyflatwasherunderflywheelnut.Youcannotusefactorycup

IGNITIONSYSTEM

1. UnalteredB&Sstockignitionpart#557125ismandatory.Only“RED”Coilallowed.Ignitioncoiloritsposition,otherthanairgapmaynotbealteredinanyway.Coilmountingboltsmustbecoiloritsposition,otherthanairgapmaynotbealteredinanyway.Coilmountingboltsmustbestockandcannotbealteredinanywaytoadvanceorretardtiming.Attachmentboltsand/orboltholesmaynotbealtered.

2. Sparkplug:Anyautomotivetypewith10mmthreadonly,unalteredwithstockwasherallowed.Indexingwasherisnotallowed.StandardsparkplugisChampionRG519HC.

3. Magnetoairgapisnon‐tech.4. Timing: Normal=29degreeBTDC

Min.=27degreeBTDCMax.=31degreeBTDC

GEARBOX1. Gear box is not considered part of the engine and is non-tech, therefore any

gear reduction box may be used.

CRANKCASECOVER1. Removecrankcasecover.

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2. Covermustbeinstock,unaltered,“ascastinfactory”condition.Noalterationsorsubtractionsofmetaloranyothersubstancetocrankcasecover.

3. Aftermarketgasketsareapproved,howevermustbeofsamesizeandmaterialasstockgasket.Onlyonegasketisallowed.

PISTON

1. Removerodandpiston.2. StockandunalteredB&S“kidneybean”piston‐part#557001only.3. Minimumfromtopofpistontotopofwristpinoncirclipsideis.685to.686in.4. Minimumpistonlengthis1.768in.5. Oversizedpistonsarenotallowed.6. Weight:Completecombinationincludespiston,rings,rod,clipcap,andbolts.:300

grams‐min.or10.6ozmin.

NOTE:Torxheadassembly‐average357gramsHexheadassembly‐average360grams

RINGS1. Mustbestock,unalteredB&Sringspart#555664only.2. Nodecreasingofringtensionbyheating,machiningoranyothermeans.3. Threeringsmandatory.4. TopcompressionringmusthavechamferorOtowardtopofpiston.5. SecondscraperringmustbeinstalledwithinsidechamferdownandOtowardthetop

ofpiston.6. Oilringmustbeinstalledasfromfactory.7. Minimumwidthoftoptworingsis.095inches.8. Thicknessoftoptworingsis.059to.064inches(eachring)9. Oilexpanderringtotaloveralllength=8.200min.10. Minimumwidthofoilringis0.65inches.Ringgroovemustbepresent.

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11. Thicknessofoilringis.098to.102inches.

WRISTPIN

1. Mustbestock,unalteredB&Spart#499423wristpinandlockpart#6918662. WristPin: MaximumI.D.=.414”

Max.O.D.=.626”Minimumlength–1.901”

CONNECTINGROD

1. Mustbestock,unalteredB&Spart#557005or555117(hexheadbolts).2. Rodlength,measurementfrombottomofwristpinholeoftopofcrankjournalholeis

2.419inchesminimumto2.49inchesmaximum.3. Diameterofbigend=27.9476mm–1.003”ref4. Diameterofsmallend–15.89405mm–0625”ref

CRANKSHAFT

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Briggs crankshaft with factory splines Briggs crankshaft with welded splines

1. StockB&Spart#555622or557137crankshaftmustremainunalteredexceptthefollowing:

a. Crankshaftmaybealteredonlyonexternaloutputshaftandonlybyaddingageartodrivethegearbox.Allothermodificationtothecrankshaftareillegal.

2. Crankshaftjournaldiameter=1.094inchesto1.100inches.3. Stock,unalteredB&Spart#555573bearingsrequired.4. B&Spart555054key–flywheel.5. Shim,(s),Briggspart#555652,ifusedmustbeinstalledasfromfactory.6. Stock,unalteredpart#695087timinggearinstalledinstocklocationoncrankshaftonly.7. Nooffsetkeywaysallowedoncamgearforcrankshaft.8. TheuseofLoctitetoretaincrankgearsecuretocrankshaftisacceptableandlegal.

CAMSHAFT

1. Stock,unalteredB&Spart#557041camshaft.2. Therewillbenoadditionsorsubtractionsfromanypartofthecamshaftexceptforthe

compressionreleasetabsasoutlinedbelow.

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Compressionreleasewillremainintacthoweverthecentrifugalforcetabscanberemovedoralteredforeasierstarting.3. Lobecenteranglewillnotbealteredbyanymeans.4. Lobeprofilewillnotbealteredinanyway.5. Nodeburring,machining,honing,grinding,polishing,sanding,mediablasting,etc,etc.

Noadditionorsubtractionofmaterialinanyformormattertocamexceptasoutlinedforcompressionreleasetabs.Unlessitsaysyoucandoityoucannotdoit.Camlodescannotbewelded.

TAPPETS1. Stock,unalteredB&Spart#557038tappetsonly.2. Tappetdiameter=.964to.984min.max.

TORQUESPECIFICATIONS

NonTech,recommendedbyB&S

FlywheelNut 55‐75ft.lbs.(74.5‐101Nm)CylinderHead 180‐22‐in.lbs.(20‐25Nm)ConnectingRod 120‐140in.lbs.(13.5‐15.8Nm)CrankcaseCover 120‐140in.lbs.(13.5‐15.8Nm)CylinderHeadPlate 70‐90in.lbs.(8‐10Nm)RockerArmstud 70‐11‐in.lbs(8‐12.5Nm)ValveCover 30‐60in.lbs(11‐16Nm)SparkPlug 95‐145in.lbs.(11‐16Nm)Techofficialshavetherighttotechanyorallcarsinanyclassattheirdiscretion.