With a diameter of 5.25", its slightly smaller than a factory crank pulley, which helps to keep from frying your water pump, alternator, and power steering pump bearings at high RPM's, as well as freeing up some additional horsepower.  These have one pulley bolt hole elongated so they'll fit on the '72 and later symmetrical bolt pattern and the earilier offset pattern.  Machined from a solid piece of billet aluminum, you can't beat these for a great combination of performance, durability and great looks!  Use this double crank pulley with NON air conditioning setups with power steering.  It also requires a double groove water pump pulley that is listed separately.