01 250F Do I need to rebuild the bottom end?

[Michael Schultz]

I tore into the top and bottom end of my YZ and the first thing I noticed was the Piston flaking and chipping, almost like it was about to explode. Anyone ever see anything like that or know if this is normal? The second thing is there is a big blue spot on the water pump side of the crank. Typically this is from excessive heat. Is this typical on these bikes? Should I replace the crank? What should I do because I have heard that these bikes have bottom problems after some time and parts could start to break causing an explosion of the bottom end. Please Help!

 

[Rich Rohrich]

The blue spot on the crank is from heating during manufacturing and is nothing to worry about. If you leave these pistons in too long they can fatigue fracture. It's tough to say without seeing the piston.

Take a close look at the gear on the left side of the crank that enages the cam chain. Look for wear on that gear and slop in the main bearing behind it. Generally it's a good idea to change the crank main bearings on 250Fs every couple of seasons on bikes that get ridden regularly.