Accurately displays piston movement. 1/8" graduation marks. Installs as easy as a spark plug. Made of 1144 steel with a stainless steel spring. Lifetime warranty; Made in the U.S.A. The TDC Indicator allows a technician to accurately set any position to top dead center. Just remove the spark plug, screw in the gauge and rotate the engine while observing the graduation marks on the sliding shaft. Note the top piston stroke.