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84 Ty 250 Topend Rebuild


m_lo
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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

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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 by feetupfun
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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

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