there are two ways to check valve to piston clearance, the most common way, and longest used, is the way 10secgoal described. the other way is basically the same except you do away with the clay, and use a dial indicator to measure the exact amount of clearance at various points along the valve timing curve. usually starting at about 20 degrees btdc, and ending about 20 degrees after. a couple rules of thumb, you need at least .080" on the intake, and .100" on the exhaust. second, if your valve lift is at or below .500" and your intake valve is 1.94" or smaller, then you should have plenty of clearance between the valve and the piston. check to be sure though.