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yz125 top end disaster

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So I completey  rebuilt the top end on my 1999 yz125 New cylinder and piston. I heat cycled it then went for a little ride not even really going into the powerband about 5 minutes later i feel it lose compression so i just let it die pulled off the cylinder and there is deep pitting on the piston and cylinder head with a single score mark on the piston very little damage done to the cylinder cant see it but i can feel it with my finger nail. IMG_0471.JPGIMG_0472.JPGIMG_0473.JPGIMG_0470.JPG

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Your piston pin circlip fell out, and destroyed everything. You have to make sure to never reuse an old one, and to always install them with the gap facing directly down or up. If they face sideways at all, they can compress and work there way out of the recess, like your's did.

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nope it wasnt the circlip i had the same pitting and big score mark on my last piston not even 10 hours on that piston both circlips were in tact and facing up

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nope it wasnt the circlip i had the same pitting and big score mark on my last piston not even 10 hours on that piston both circlips were in tact and facing up

My next guess would be that your air filter is not filtering at all, and is letting debris pass right thru. Either that, or you have a big end/small end bearing going out, and the needles found there way up thru into the top-end.

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Nvm I know what that is I put a new small end bearing in along with the piston it's not that it could be the big end bearing I will have to check out the crank thanks for the reply helps a lot

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Is the pin that aligns the rings still in place?

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Looks likes the crank is going out and is sucking pieces onto the piston.

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