This is the HPI 12T Clutch Bell. This Clutch Bell is compatible with the HPI Bullet MT 3.0 and Bullet ST 3.0. Change the gearing of your Bullet MT or ST for increase performance! With a smaller clutch bell, your acceleration off the starting line or out of a turn is greatly increased; with a larger clutch bell you'll get an increased top speed! It's your choice and they're simple to fit!