That is a Traxxas 52T, 32P Spur Gear to be used into the center Differential of the Slash 4X4. Features: M0.8 pitch 52 tooth Ebony synthetic construction Appropriate for 32P pinion gears Includes: One 52T Spur Gear Specs: Pitch: M0.8 Enamel: 50 Outer Diameter: 43mm (1.69") Inner Diameter: 22.5mm (0.89")