This is an optional ProTek R/C Hardened 13 tooth clutch bell. These clutch bells are hardened and precision cut, providing you with worry free performance that is on par with any manufacturer. The clutch bell is installed over the clutch shoes and will engage when the engine rpm's increase, allowing the engines power to transfer to the center spur gear and the rest of the drive train. A clutch bell with fewer teeth than what you are using will provide increased bottom end acceleration with reduced top end speed, which is good for smaller technical tracks. A clutch bell with more teeth will increase top end speed, at the expense of reduced bottom end acceleration, which is good for larger flowing tracks. Includes two rubber sealed 5x10x4mm clutch bearings.