This is the optional Extra Hard 15 Tooth Clutch Bell from Robinson Racing for the Traxxas Revo, and 3.3 Revo.This clutch bell attaches to the engine crankshaft, makes contact with the clutch shoes and meshes with the main gear to power the vehicle.FEATURES: Constructed of hardened steel and precision CNC machinedINCLUDES: Extra Hard 15 Tooth Clutch Bell for the Traxxas Revo REQUIRES: Installing onto clutch assembly Two 5mm x 11mm bearings (can use DTXC1547) SPECS:Diameter of Toothed Area: 0.68" (17mm)Inner Diameter of Bell: 0.93" (23.7mm)Thickness: 0.65" (16.4mm) COMMENTS: Other clutch bells in this series include the 16 tooth (RRPC8016)and the 17 tooth (RRPC8017).