Rebuilding the engine on my YZ400. Any suggestions?

I plan on replacing the rings and piston along with doing a valve job. Should I have anything else done at this time? The bike runs great, there is just a lack of compression. Still not bad for an original 99. I will be having a reputable mechanic do the work.

Just to prevent a headache, I'd pull the two side covers. Pull the clutch side to inspect the clutch basket for cracks and with the access replace the counterbalancer key. The key is about $2 and is a major cause of engine knocks. With the ignition cover off pull the flywheel and remove all the iron sludge stuck to the magnets.


I would check the big end for play by cleaning the oil out of it by flushing petrol or degreaser through the oil way at the end of the crank so that you get an accurate feel of the play and it is not cushioned by oil. Also check the small end for scuffing on the conrod. I have heard that the small end can wear and cause the rod to break, which can distroy the engine!

I would also change the water pump seals and shaft.

Good luck.

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