When pullinbg me head on the new motor to replace a oil galley plug and headgasket I noticed some of the pistons a round scuff on them. Im sure this is a valve toughing the piston. You cant grab an edge of the scuff with your fingernail so its very very light but its still there and only one valve per cylinder is touching. How do I check piston to valve clearance? Specs on motor are in sig