This is the Hard Aluminum 48P 39T Pro Pinion Gear from Robinson Racing.FEATURES: Hard Aluminum Construction Black in Color 39 Tooth 48 PitchINCLUDES: One 48P 39T Pinion Gear One 3x3mm Set ScrewREQUIRES: Installation on the Motor ShaftSPECS: Outer Diameter: 21.65mm (0.85") Inner Diameter: 3.15mm (0.12")