My 66 has a double groove Ford pulley on the alt, crank, and water pump, and is actually running 2 redundant v-belts. I had thought this was because at some point the car probably had power steering and/or AC (not anymore). The guys at my shop tell me it is to keep a thin v-belt from being tossed from a single groove pulley at high rpm's. Also it helps keep the belt from slipping when turning a tight accessory... I have no idea, but if you are buying pulleys might want to consider it. Anybody heard of this?