Pull Start One Way Bearing Fit for HPI Savage Firestorm Trophy, 12mm Hex 6mm Shaft Fits HPI, Kyosho and other engines, but check measurements to ensure it will fit your pull start and engine shaft. This product is made from high quality materials, and designed for lasting performance It is 12 mm hex with 6 mm shaft fitment Not For use with Electric Roto-Start Style Systems NOT FOR THE NEW F4.6 Nitro Star Version 2 Engines