Yz450f 2005 Engine rebuild: Advice needed!

Hi guys I recently swapped a KX 125 for a Yz450f 2005 that had locked up at low RPM's in first gear (Or so the guy says) the guy thought it had probably dropped a valve but upon further inspection, all valves where fine and un damaged along with the piston and the cylinder wall, what we found was the crank had seized causing it to have no backwards or forwards movement but still allowing it to move up and down slightly within acceptable tolerances, what I am wanting to ask is whether or not I will have to replace the top end too aswell as the bottom, the cylinder and piston are in good shape and if I was going to replace the piston it would be another £100 and then id have to get the cylinder honed again. Is it worth getting a fresh top end to match a new crank and bottom end bearings?

Cheers - Lewis

The top end is reasonably cheap by comparison.  If the rob bearing seized, the piston was probably distorted, and should not be trusted.  I would, at the least, replace the piston and rings.  Honing the cylinder with a "spring-ball" or "brush" hone just quickly enough to restore the original finish is advisable, but you could spend a little time with some Scotch Brite if you want to cheap out on the job.  Results aren't as good but it works, sort of.

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