PISTON / BARREL DIFFERENCE
can some one tell me what the piston in barrel tolerence is , ie if the piston was 62mm what would the barrel be , as from new ,thanks
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bristolmod
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- Main scooter: Lambretta TV175 S3- what else!
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depends..............
Generally pistons in cast iron barrels have 3 1/2 thou clearance (we're talking "old" money here NOT metric). Some people would recommend 4 thou as a minimum.
Alloy barrels can take a "tighter" clearance- I've done 3 SR 185 kits (steel lined alloy barrels) and I've worked down from 4 thou, through 3 1/2 thou to my present one on 3 thou. The 4 thou one rattled, but the 3 1/2 thou and the 3 thou ones were fine. No nip ups on any of them.
I would recommend the services of a reborer who knows what he is doing- he will then bore the barrel TO the piston to ensure the correct tolerance. Whilst you state that the piston is "62" mm, this is unlikely to be spot on, as pistons do vary (and indeed are made with a different measurement at the crown as compared with the base)
Chris
Generally pistons in cast iron barrels have 3 1/2 thou clearance (we're talking "old" money here NOT metric). Some people would recommend 4 thou as a minimum.
Alloy barrels can take a "tighter" clearance- I've done 3 SR 185 kits (steel lined alloy barrels) and I've worked down from 4 thou, through 3 1/2 thou to my present one on 3 thou. The 4 thou one rattled, but the 3 1/2 thou and the 3 thou ones were fine. No nip ups on any of them.
I would recommend the services of a reborer who knows what he is doing- he will then bore the barrel TO the piston to ensure the correct tolerance. Whilst you state that the piston is "62" mm, this is unlikely to be spot on, as pistons do vary (and indeed are made with a different measurement at the crown as compared with the base)
Chris
Scootering since 1968.
no your right,the piston is stamped 62 std but it is actuly 62.14mm
take your piston to the re-borer with the barrell , and ask for 4 thou clearance on a cast barrell, and 3 thou on an ali /cast liner. i get mine done at FRONTLINE MOTORCYCLES, in southbank in middlesborough, and he is spot on. had dozens done, and he does my cranks, and a lot more. proper motorcycle engineer , who likes lammys, but can do barrels in his sleep. don"t expect to get a smile though! 
i get all new cast barrel kits checked out, regardless of make, the extra ten or twenty quid is worth it, and ,touch wood, they benefit from it......... with little or no comebacks.
i get all new cast barrel kits checked out, regardless of make, the extra ten or twenty quid is worth it, and ,touch wood, they benefit from it......... with little or no comebacks. thanks
hi i am taking all three to the machine shop what is it 3thou or 4 thou ?, cast barrel and cast rings, i dont want any piston slap ,