This is the Robinson Racing Hard 5mm 48P 24T Pinion Gear.
Extra-hard high carbon brushless motor pinions are built to take all the power you can deliver. Can be run on cars using an 5 mm motor shaft. One 5x5 mm set screw included. 2.5 mm allen wrench (not included) fits the set screw.
One Robinson Racing Hard 5mm 48P 24T Pinion Gear