One-Way bearing for the HPI Nitro Star 21BB engine. Also fits the Roto Start for the Savage. This One-Way Bearing allows the recoil starter to grab the crankshaft to start the engine and spin freely in the opposite direction. FEATURES: Steel construction YOU WILL RECEIVE: One one-way bearing One installation instruction sheet REQUIREMENTS: Installing in recoil unit SPECIFICATIONS: Overall Length: 12mm (.472") Hex Width: 12mm (.472") Inner Diameter: 6mm (.218") COMMENTS: If recoil unit is needed, use HPIG1428 description revised by: DB February 21, 2013