Yamaha YZ450F Full or Part Engine Rebuild?

First time posting on here.

Me & My Son recently acquired an '05 YZ450F with a seized engine. 

On further inspection, it turns out the crank has seized but the rest of the engine is perfect.

My question is should we replace the top end as well as the crank? The Piston, Bore & Valves etc are in perfect condition, zero scoring.

I'm happy to replace the Crank while my Son is insisting we do everything (I'm paying for it all btw).




Check the oil pump out.I'd throw a piston kit and timing chain in there aswell.if the valves and head look good and the clearance is still in spec I'd run the head

Timing chain just whenever you open the engine up.  Obviously, a complete new crank.  If the bore is in fact in good shape clean it up with a bottle brush hone and replace the rings. 


Considering that the crank seized, that means that the force of stopping the engine was applied to the piston, and for that reason alone, I would replace it.  Check the oil pump, and the condition of the trans bearings and the bearings that carry the balancer shaft. Replace the mains.


You might also want to check the condition of the valves.  Reject any that have detectable wear on the face (any sort of ridge at the edge of the contact marking.  DO NOT EVER lap titanium valves, no matter what the book says, and avoid running a wire brush against the faces while cleaning the rest of the valve.

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