Update your old 4-speed Big Twin from the old coil spring clutch to the late style diaphragm spring clutch which proved to be superior right from the start when it was introduced in 1985 on Evo Big Twins. But the Pro-Clutch has more to offer and also Evolution owners can achieve major improvements by its installation. The clutch area is increased by almost 100%, which allows a decrease in diaphragm spring pressure. For the rider this means less effort at the hand lever, smooth control and still excellent horsepower handling potential. Repeated burn-outs or two-up serpentine riding in the mountains with frequent clutch dragging are no problem for a Pro-Clutch. All kits install quickly and easily into the stock chain housing. The kits for 4-speed Big Twins from 1936–84 come with a durable double-row ball bearing that replaces the shaky stock “caged bearing” for a sturdy connection between mainshaft, hub, and basket. The stock clutch basket may be retained. Kits for Evolution models are front chain drive only and are also designed to work with the existing OEM clutch basket. All Pro-Clutch kits include a clutch hub, friction plates and steel plates, stainless steel pressure plate, diaphragm spring, spring retainer, four shoulder nuts, a clutch hub nut with seal, and installation instructions.