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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.


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.


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.


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


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.


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.


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

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


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.


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


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.


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


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.


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.


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


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.


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.

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