This t.d.c. Indicator is needed when assembling any engine. It indicates top dead center (t.d.c.) position of a piston on any gas engine. Simply unscrew a spark plug and twist the rubber hose part of this tool into the spark plug hole. As you rotate the engine with a turning bar, the piston forces air through the whistle part of this tool. Once the whistling stops the engine is at t.d.c.