Note: This JEGS 20452 Timing & Gear Set for 429-460 Big Block Ford includes a 3-keyway performance crankshaft sprocket. The standard timing mark is 0 and is for a factory emission-controlled engine with an 8-degree retarded timing dot. (Do not use this setting when installing a performance camshaft). A for advanced 4- degrees valve timing and R for retarded 4-degrees valve timing. When installing a performance camshaft, it is recommended to degree your camshaft for the best results.