Gear, 21-T pinion (48-pitch) / set screw. Optional Traxxas pinion gears allow you to increase or decrease the final drive ratio and tune for different applications. Larger gears will result in higher speeds while smaller gears provide more torque. These are designed for use with 48-pitch spur gears and 3mm motor shafts. Use Traxxas stock and hop-up replacement parts to get the most out of your Traxxas RTR vehicles. This product is made from high quality materials and designed for long-lasting performance. This is replacement hardware for use on Traxxas vehicles; consult your user's manual for exact parts listings.