Just a couple of $.02 advice from one that has spent much time with a degree wheel, dial indicator, and clay on top of pistons. First don't use clay but the much less messy Play-Doh. Second, valve-piston clash almost never occurs when the piston is on the way up or at TDC but rather the valve catches up to the piston on it's way DOWN. This can be from not enough deck height, pistons with no fly cuts, wrong cam with wrong lift amount or rate of lift (lobe profile), plus several other possible problems. For checking an assembled engine one can make up a lever to work the rockers since Stovebolts have very weak springs or even better is to pressurize the cylinder, pop the keepers off the intake and exhaust? (recessed) and then reassemble with very weak springs; this lets one work the rocker by hand. A 2' diameter degree wheel and a bolt in the front of the crank lets one move in very small increments. With a magnetic base dial indicator one can check the piston to valve clearance every 2* from 30* before TDC to 30 after in a short time. As was stated, any cam that needs piston to valve clearance checked on a Stovebolt engine is a waste of money since the head and manifold flow about the same as a flathead Briggs and Stratton.
