The ARRMA 13T 0.8Mod Safe-D5 Pinion Gear  is a heavy-duty steel pinion gear featuring a 13-tooth count and 0.8 Module (32-pitch equivalent) gearing. 
It is specifically engineered to fit 5mm motor shafts and is highly favored in the RC community for its unique "D-shaped" bore design. 
This profile mechanical locks the gear to the motor shaft, preventing a complete loss of drive and braking power even if the set screw backs out.
Safe-D5 pinions only slide onto motor shafts with a 5mm diameter and a flat spot; they will not fit 3.17mm (1/8") shafts or purely round 5mm shafts without a flat landing.