CNC aluminum quality construction designed to fit 10mm spark plug holes used on Zenoah, CY, and similar clone engines. We designed this tool to lock the piston just after "bottom dead center" at the strongest point so less load is placed on the piston rod. Simply screw it in the spark plug hole and it will lock the crank shaft so you can install or remove; clutches, collets, and flywheel. Non-slip knurled handle, a must for all workshops and tool boxes.