The double keyway option requires that the factory or aftermarket crankshaft be removed from the engine and a second keyway cut into the snout of the crankshaft. DO NOT run a double keyed damper on a crankshaft without the double keys. Failure to the damper hub can happen without the supporting key in the broached keyway slot. The dual keyway option is for Supercharged or Extreme Duty Applications (typically over 750 rwhp). Without the double key modification your crankshaft in these high horsepower applications can lead to the failure of the damper hub and/or the single keyway which results in damage to the damper and the crankshaft snout.