What PSI should i be running?

[Hondamaster5505]

When we eventually get time and do my valves, i also wanna get a compression piston and cam.

We're gonna do a compression test, and if its low or on the low side, we're gonna go .020 over. If its good, we're gonna get a stock bore piston.

So, what psi should a stock 400ex be running? And whats the wear limit for it?

[drew416ex]

Stock compression should be between 100-128 psi. New piston to cylinder clearance is .0006" - .002" . The service limit is .004" so you will most likely need a slightly larger piston if you go by the service specs which many dont on this forum.