This pinion gear is designed to fit the newest versions of the Trex 450 helicopter that come equipped with a MOD 0.6 helical cut main gear. The pinion has a 3.17mm center hole that is designed to fit the Scorpion HK-22mm motors, or any other helicopter motor that is equipped with a 3.17mm shaft. The pinion gear is made from hardened aluminum, and comes complete with a set screw for attaching it to your motor shaft.