Crank straightness is easy to check. Put the engine/plane securely somewhere that it can not move at all. Turn the prop to any position and move an object up to just touch the blade. Rotate the prop carefully (don't move plane/engine) and see if the other blade tracks to the exacts same place. You will have to put the prop in several (3 should do) positions on the crank to verify straight but ANY failure is crooked. Just be sure you don't move anything but the prop. Would be easiest w/o the plug installed. Don't have a dial indicator I suppose