This is the Tekin 13.0mm "High Torque" Sintered Brushless Tuning Rotor with a 5mm Shaft, and is intended for use with the Tekin SC4X family of brushless 550 motors. This rotor is a great upgrade for any Tekin SC4X motor that has a 12.5mm rotor. It will provide more bottom end and cooler running temperatures compared to the 12.5mm rotor.