09 250 with piston/cyl damage with pics

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?

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Hard to see,but the looks of it Piston tried to exit via exhaust port.

Common on these when the cylinder gets worn.

Id still think detonation..

Exhaust side...very common.

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.

Do I see a ring gap?

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

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.

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?

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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.

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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