New ProForm piston top dead center tool aka «piston stop». The purpose of a top dead center/piston stop tool is to ensure the motor is at TDC when installing a distributor, checking balancer marks accurately or adjusting timing tabs. This threads into the spark plug hole on 14mm cylinder head holes so you can find top dead center. There are several ways to use this tool, the most common method is to install the tool in place of a spark plug on the #1 cylinder, thread the stop until it just touches the piston, mark the balancer at a reference point, rotate the motor by hand backwards CAREFULLY until the motor stops against the piston stop, mark the balancer again at the same reference point, and the halfway point between your marks is your TDC (Top dead center). This will work in the following motors below, as well as other 14mm spark plug holes. This tool should only be used with hand rotation of the motor to prevent damage to the piston. For certain dished piston applications and certain angled head applications, we suggest the longer stop version (ITEM# 125545111252). 97-13 LS based engines 67-97 Small Block Chevy engines 67-95 Big Block Chevy engines 67-81 Pontiac 301-326-350-400-455 engines 77-79 Oldsmobile 403 engines Also other 14mm hole engines.