Product details:

Professional Products 6.7 Inch Damper for Big Block Ford 429-460 has a damper that features the stock 3/16 inch keyway, plus it has a 1/4 inch keyway 180 degrees from the stock keyway. This allows the damper to fit certain aftermarket dual keyway crankshafts. Although this engine is classed as an externally balanced engine, the counterweight is not part of the damper. It slides onto the crank before the damper is installed. The standard counterweight is Ford M-6359- D460 and must be used unless the engine has been converted to an internal balanced assembly. The damper itself matches the original equipment damper dimensions.