Insert the Flexible whistle alert into any spark plug hole and bump the engine until you hear the whistle blow. You are now on the Compression Stroke!. Easily finds top dead center on compression stroke - quick and easy to use. Rubber nipple adapts to any spark plug thread size. Flexible hose allows for reach in tight spaces. Made and assembled in U.S.A. No more burning fingers and hands. Just listen for the whistle to find the TDC Compression Stroke.