Sim­ple solutions:

Re-bores need the right tools and ex­pert knowl­edge!

Classic Motorcycle Mechanics - - WORKSHOP -

3/ And swear… loudly! The bore is al­ready at sec­ond over­size i.e. 0.5mm and as we know the dam­age goes deeper. There re­ally is no point in re-bor­ing un­less there’s a pis­ton in stock to suit. 4/ In an ideal world these third over­size pis­tons would be per­fect but our bar­rels are so badly worn in places they re­ally need to be ma­chined to fourth over­size or 1.0mm…so more ex­pense! 5/ Al­ways check the di­men­sions of re­place­ment pis­tons; never as­sume any­thing. It’s not un­com­mon to find the wrong sized pis­tons in cor­rectly marked boxes. And very oc­ca­sion­ally the pis­tons are ac­tu­ally stamped up wrong; mea­sure twice, cut once etc. Al­ways take the mea­sure­ments 10mm from the bot­tom of the pis­ton front to back, not side to side. 6/ The cylin­der is po­si­tioned on the bed of the bor­ing bar with short, over­sized, steel cylin­ders top and bot­tom as sup­ports. SP A bore cylin­ders from bot­tom to top; this keeps the line of the bore per­pen­dic­u­lar to the base gas­ket/crack case in­ter­face en­sur­ing cor­rect align­ment to the crank. 7/ The bor­ing head needs to be cen­tralised in the cylin­der or we’ll just be caus­ing car­nage here. The three pins, lo­cated at 12 0 de­grees from each other, fa­cil­i­tate pre­cise cen­tring.

8/ With the bor­ing head set and the tool in place the cylin­der is care­fully bored and gen­er­ally the cuts are made in­cre­men­tally if there’s a lot of metal to take out. ob em­ber­ton bores to one tenth of a thou whereas the fac­tory specs set the limit at two thou! This is just one of the dif­fer­ences be­tween a quick re­bore and a proper job. 9/ ost bor­ing bar the freshly ma­chined lin­ers will need hon­ing; if this isn’t done the rings will get a re­ally rough time. Hones come in a range of var­i­ous sizes to suit dif­fer­ent bores. They also run dif­fer­ent con­fig­u­ra­tions of stones or cut­ters. One cru­cial el­e­ment of two-stroke re-bor­ing that has been omit­ted is port cham­fer­ing. Ev­ery spe­cial­ist has their own tricks of the trade and ve been asked not to pho­to­graph Rob' s tech­niq ues but trust me when say they are done and done prop­erly. f the ports aren' t cham­fered then the pis­ton rings will make dra­matic, sud­den and hard con­tact with the port edges which is guar­an­teed to do dam­age. With de­cent two-stroke oil in the tank, al­low­ing the en­gine to warm up when cold and a mod­icum of throt­tle re­straint as the mo­tor gets to op­er­at­ing tem­per­a­ture, the re­bored bar­rels of Pro­ject amaha CS3 C should last for ages. ou' d doubt­less nd some en­gi­neer­ing shops of­fer­ing 15 re­bores as op­posed to the 5 VAT SPA Mo­tor­cy­cles charge but get thisº a uick re-bore and a ca­sual hone are of­ten only one step above the knack­ered cylin­der you just handed over. Surely your mo­tor is worth more than that? 10/ We’re us­ing a four stone hone on the two-stroke bar­rel to aid the cut on the base or spigot area in­di­cated. With a two stone hone there’s a dis­tinct pos­si­bil­ity the forces in­volved might de­form the un­sup­ported liner which is sur­pris­ingly eas­ily dis­torted. 11/ Af­ter the bar­rels have set­tled overnight from the re-bore they’re honed close to size, then al­lowed to sit again to re­cover. e-honed a sec­ond time, they may get an­other ses­sion if nec­es­sary. Overkill pos­si­bly? Not when you re­alise ob builds top line racing two-stroke en­gines! 12/ rior to fit­ting the bar­rels, get an in-depth wash in hot soapy wa­ter which might sound like mad­ness but it’s not. very last frag­ment of re-bore and hon­ing de­tri­tus needs to be evicted or there will be ring/ pis­ton/bore wear: post de­wa­ter­ing with some iso­propanol, the bores are coated with ACF -50 to keep cor­ro­sion at bay.

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