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My bike overheated in the last mud race, tore a radiator hose, lost coolant, blew a head gasket. I finished the race, maybe I should have quit. I washed it and started it after the race and it ran rough and had this terrible rattling noise when I raised the RPM's. Pictures of the piston, crank, etc are below. The cylinder looked fine so I lapped the head and top of the clylinder and re-built the top end. I started the bike up and hoped everything would be good. It runs much better and sounds smooth at idle but when I raise the RPM's it still has that bad rattling/vibrating sound. You can listen to how it sounds in the links below.





I looked high and low for something external that might be causing this. Anyone have any ideas? Could it possibly be the crank?

lower rod bearing? that would be my guess.

crank bearings


Did you spin it over a few times while holding on to the rod?


Should have been easy to feel and hear if it's crank bearings

I would have the crank rebuilt (rod kit w/lower rod bearing and pin), and replace the crank bearings/seals while everything was apart. IMO, that is the only way to approach this job. You may as well pop in a new piston/rings while you are in there. Then, at least you will know where you are, engine-wise.

The discoloration on the rod is normal, and your piston has a little bit of blow-by, but again, it looks pretty normal (for a well used trail bike). It is very difficult (impossible) to identify these types of problems visually. Does the rod have *any* up and down movement at all?

If your gonna split the cases dont forget to replace the crank seals.

that piston doesn't look bad, about the same amount of carbon deposit and blowby I had on my last top end.
It sounds really rough though, my vote would go toward crank bearing too.