1.610" ID with 3/16" keyway. Engine work requires repeated crankshaft rotation. This non-marring, aluminum tool rotates the crankshaft the proper way, when the balancer is off. Making sure to have the proper size, position this socket over the end of the crankshaft, so it can be turned using a 1/2" breaker bar or drive ratchet. Fits 1965-1992 Chevrolet models.