yz 400 engine trouble - aluminum in oil

I have a 99 yz 400 - after a long hot trail ride a couple weeks ago I was quite disturbed to find the oil was full of aluminium powder and flakes. After that I no longer needed to pull the decompression lever to kick the bike over. The engine still runs, but sounds awfull. I don't want to run it anymore, I guess it's time to pull the engine apart and find out what went wrong. Has anyone had this happen, any ideas what parts I might need to replace/repair. I have never pulled it apart, but I have the shop manual. I guess I'm out for the season.

thanks in advance


Possibly bearing material from a bad bearing.

Or it could be clutch material. But if the engine isnt running right anymore, then its probably a bearing failure.

Are you sure it's aluminum? If it were a bearing failure the bits and pieces would be mainly steel. Aluminum is far too soft to be a bearing material. My guess (total guess) would be a scored cylinder/piston, and the aluminum is bits of the piston skirt, if of course the piston is aluminum, is it? That could explain compression loss (past the rings) and running badly.


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