Classic Dirt Bike — Autumn 2017

runin steelrings,Elecktronnotbeinganideal

materialforthehousing.Theseringsshouldbea

goodfitinthecase,butifthere’sanysuggestion

of mainshaftmisalignmentonthecrankthen

thiscanloosentherings.Goddensuggested

Aralditeadhesiveto tightenthemupagain.On

themainbearingsGoddenadvisedit wasbest

to renewthemeachracingseasonor problems

wouldoccur.Also,thecrankpinhadalimited

racinglifeandtheadviceherewastojunkit

after 25 meetings.Thepinis wherethebigend

runsandhasahardenedsurfacefortherollers.

Underracingconditionsthissurfacebreaks

upandallsortsof nastythingscanhappen.If

thebigendhasfailedthenthecausemustbe

investigatedor it willhappenagain–itcouldbe

theoilfeed,or ablockagesomewhere.

Toreplacethecrankpin–andthisis an

indicationof howenthusiastsseemedto be

muchmorepractical 50 yearsagobecause

thesedaysthecrankwouldbeshippedoffto

aspecialist–thearticleadvisedto holdthe

flywheelbyclampingalargespannerin avice,

sitthecranknutin it andheaveontheother

butwithasocket groundto removethelead

in radius.Thenusethreeboltsto putpressure

ontheinsideof theflywheelsandthenfixan

oldcrankpinto theoriginalannndgivetheenda

sharptapto releasethetaper.AAAstimegoeson

theflywheeltaperscanenlargeee,soJAPprovided

oversizecrankpinsto copewittththisand

allowthepropersidefloatof thhherodto

beachieved. Assemblingwasrrregardedas

asimpletaskmerelyrequiringanew

crankpin to beboltedin place,,,the

rodwithnewbearingrollersfitttted

thentheotherflywheelfittedannnd

thewholelotboltedupwithneeew

nuts.Anymisalignmentfound

in thetruingjigwascuredby

bumpingtheassemblyonastooout

surfacewithasheetof softmetttal

in place.Willoughbysuggesteddd

thebalancefactorshouldreallyyybe

checked, thoughonlyas acourrrtesy

as it wasunlikelyto befarout –

butyoucanbetGoddenalwaysss

checkedhis.

Goddensteppedawayfrom

standardJAPpracticeanduseddda

Mahlepistonwithathicker crooown

whichwasusefulforcopingwwwith

highliftcamswhichwouldneeeed

ABOVE:Donaimedtohavethecrankrunningastrueas

possible.Ifthey’renotthentheridecanbeuncomfortable

andit’snotgoodforrace-winningpower,plustheengine

canshakeitselftobits.

ABOVE:Eachcamis foradifferentpurpose,withgrass,

sandandlongtracksrequiringslightlydifferentpower.

Allcasefacesareprepared

sotheymatefullyand

preventleaks.

BELOW:If youthinklogicallyabouthowmanytimesthe

rockersopenandcloseavalveandhowheavytheyareas

standard,thenlighteningthemmakessenseasit givesall

othercomponentsaneasytime.

ABOVE:Cleanandtidy, notoverlyglitzy,andblisteringly

fast...abit liketheriderreally.

thevalvepocketsdeepeningso thevalves didn’t

cloutthepistononfulllift.Alsoadeparture

fromJAPpracticewastheclosing-upof the

ringgaps.Mostpeoplewillknowthepiston

ringsformpartof thesealingof thecombustion

process,andtheringgapisanimportantpartof

that.Toobig agapandtheengineburnsoil,too

smallagapandtheringendswilltouchwhen

theengineis hotandseize.Usingalcoholas a

fuelmeanstheenginerunsalot cooler,andDon

foundasmallergapworkedperfectly.

Naturallywithamandedicatedto

performance,therewerealsoafewtipsof use

to thoseworkingonabudgetandif replacing

standardcamswithhighperformanceonesisn’t

viablethenperhapsGodden’singeniousmethod

of grindingbacktherocker padonthearmmay

beof interest.Also,Godden’smethodof timing

theignitionis sureto createsomediscussion;he

didit byear!

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