Designed specifically to fit under stock or aftermarket primary/derby covers. Increases the capacity of your stock clutch and keeps lever effort to a minimum even with big inch motors. Stainless steel weighted arms progressively increase pressure to the clutch as RPM's increase. Three different sets of six coil springs (light, medium, heavy) are included for a wide variety of spring pressure options. The coil spring design provides smoother shifting and a more linear engagement compared to a diaphragm type spring.