October 13, 201213 yr Hey Guy's I'm looking at rebuilding the topend of my o'l ty250,just a question ( remember that I'm not a mechanic ) The top of the piston has two marks stamped on it : an arrow that points to the exhaust which obviously shows position of the piston and the other mark is a small > any idea what this indicates ,oversized piston maybe ? I bought this bike a couple of years ago and until now have never had it apart ,the previous owner did'nt know much about the bike either; other than he thought that it may have been punched out at some point ... All ideas and suggestions on my little project would be greatly appreciated Thanks M_lo
October 14, 201213 yr oversize pistons usually have numbers stamped on the crown indicating the size that piston is bigger than standard size. The numbers could be imperial or metric values. If it says 010 it would usually mean 0.010 inches over standard size. If it says 25 that would probably be 0.25mm over standard size Pistons made for Yamahas would usually have the piston oversize in mm units Standard size pistons do not usually have size numbers stamped on them Sure-fire way to work out if it has been rebored is by measuring the bore diameter No idea what the > might mean Edited October 14, 201213 yr by feetupfun
October 16, 201213 yr Wasn't there two typres of piston for these, an early and late piston? . I seem to remember the later model piston had two recesses in either side of the piston head to help the swirl and the earlier one didn't. As Feetup writes, you need to measure the old piston and measure the bore too, then work out what size up you need to go (taking into account the piston clearance) as a really worn/scored bore may need to go out past the next size up. Most two stroke/motorcycle engineers will be able to do this correctly for you. Genuine pistons are getting thin on the ground, so it's worth getting an opinion of someone in the know about which to use. Maybe these can help http://www.bjracing.com/ty350.html
October 19, 201213 yr The late piston is from a Pinky with the recess. This is the one to buy as it works much better than the earlier
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