Getting ready to make a yamaha 350 a 400. I already purchased a 2000 400 crankshaft and counterbalancer from a 400 (it was cheap and new), and a 2001 400 cylinder. I also bought the 400 timing chain as well. I cannot find a high compression (10.5:1) for a 400, but they make them for 350. In the wsm catalog 350/400 pistons are the same part number , but are stock /low compression. So I should be able to use the 350 hi compression piston in the 400? Remember, the 400 has a shorter rod to compensate for the increased stroke.

 

Sneaky 401cc stroker- the easy way and more. - WolverineCentral.com Forums

I am going to be doing the same come winter. GL with the build and please post pics here and let us know how it goes.