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09 250 with piston/cyl damage with pics

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Ok guys, 09 YZ250 with about 25 hours since topend, bike was running great with no indications of a problem....until this morning:) Running 93 octane with Amsoil 32:1. No detonation etc....plug is a nice coco tan. PC Platinum and V3 reeds.

so what do you think?

DSC_0137.jpg

DSC_0134.jpg

DSC_0133.jpg

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When you say "top end" was that done with a new cylinder or just a piston?

The small bridges between the three exhaust ports wear really fast,while the rest of the cylinder will look fine.

In the end...the ring will catch.

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Id still think detonation..

Exhaust side...very common.

I did the same thing to my 09 two times.

Race gas seems to have solved the problem.

Edited by jnr450r

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iv seen to much problems with 2 ringed piston than one. said that 2 ringed pistons generate twice the heat but twice the cooling. could be true but i think it generates heat faster than it cools down therfore the catching on the exhaust port.

go with one ringed piston on your next rebuild.

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When you got the new piston did you get the right piston? There are 4 cylinder / piston sizes, A, B, C, D. Did you just get a generic YZ250 piston or get the one to match your cylinder?

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When you got the new piston did you get the right piston? There are 4 cylinder / piston sizes, A, B, C, D. Did you just get a generic YZ250 piston or get the one to match your cylinder?

...

0.01mm is not really that bigof a deal. you could use a D size on a freshly plated cylinder as per manual and still will not do any harm. that is if used cast pistons.

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On my '00 when I used a B piston in my D cylinder it rattled something awful. After a couple rides I figured out what it was and ordered a D piston. No more rattle.

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When the piston is replaced, you can safely put a D in just about any cylinder. The A,B,C,D tolerances matter on new engines. Not so much on ones with some hours on them.

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