Got so I have been rebuilding my 99 Ls1 and started to clean the pistons and noticed the pistons have been hitting the valves so I need new pistons but don't know how much valve relief I need how do I figure this out ? Or any recommendations ? Brands and sizes here is the info on the motor 99 Ls1 has been honed it's getting a vengeance vrx4 cam and 799 heads was hoping for 11:1 compression

 

Most aftermarket forged pistons have more than enough valve relief depth unless you're using a big solid roller or way bigger valves.

 

You won't know until you put in the new cam, put on the 799 heads with a used head gasket, use one of the old pistons and measure piston to valve clearance. (Use a test spring) If the valve hits the piston, get a flycutter and make a relief until you get the proper clearance.
If you buy aftermarket rods, start over. I can't imagine that cam needs reliefs though.

 

I was hitting my valves with stock can and heads