Getting a piston for my yz and It says power valve must be modified for .040 or larger soo what does that mean and how do you do it.?
The power valve is the little metal flap that cover up part of the exhaust port while the engine is operating at low and mid RPM ranges. If you have bored the cylinder and the power valve is protruding into the cylinder, then you'll have to machine off the power valve or use a die grinder (dremel tool) to provide additional clearance. EDIT - If you're ordering the piston that is .040 oversize, then the power valve has to be modified. If you're ordering the .030 or smaller oversize, or the stock size, then no modification is needed.
If you are replacing stock with stock, you don't. I wouldn't. That bike is powerful enough without a bore kit.
