Competition Cams 4797 Crankshaft Socket for Big Block Chevrolet For Chevrolet Big Block 1.610 inch ID with 3/16 inch Keyway Specially designed 1/2 inch drive socket allows rotation of engine assembly Knurled retaining nut that holds degree wheel in place Can be loosened while on crank snout so that wheel can be turned independent of the engine when finding TDC