Pinion gears are attached to the motor shaft and mesh with a "driven" gear (e.g. crown, spur). Pinion gears are available for 2.0mm motor shafts (most common) and smaller 1.5mm motor shafts. You will also need to know the diameter of the pinion required to mesh properly with your driven gear and the number of teeth for your desired gear ratio. To prevent damage to the motor, always use a pinion puller when removing pinions and a pinion press when installing them.