UPDATE: There is a new heat treated hardened hub available now for this clutch. The Hilliard 5/8 bore with built in key will shear off when used on a motorized bike. It will happen, it appears the key is not heat treated and is brittle. The cycle of on and off throttle will hammer the key and it will shear off. It happened to me and numerous others. So beware the clutch will fail. Update these clutches will eventually break, they'll shear the key off, buy Max Torque instead they will last. The Hilliard will break. It is a shame they won't fix this design flaw, because they work well and smooth until they don't. Okay new update, bmikarts.com has a replacement heat treated hub for this clutch for $16.95