The 27-tooth pinion gear (1.0 metric pitch) is machined from hardened steel for extra durability and fits 5 mm motor shafts. 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. Includes set screw.