The heavy-duty 21-tooth (32-pitch) pinion gear is machined from hardened steel for extra-long life. The gear fits 3 mm motor shafts and includes set screw. Traxxas pinion gears let you fine-tune the performance of your Traxxas model. Changing gear ratios can help balance acceleration and top speed for different running conditions. Please consult your owner’s manual for more information on recommended gearing options for your specific model.