This is an Adapter Kit Used To Mount Tru Turn Aluminum Spinners Onto The: OS Max 120 & 160 Flat Twin and Flat Four 240 & 320 Engines. FEATURES: Steel Adapter Nut Screws Onto The Crankshaft Holding the Propeller On, Then A 10-32 Bolt Screws Into The Nut From Front, Holding The Spinner In Place. INCLUDES: One Long Adapter Jam Nut One Aluminum Bushing REQUIRES: 1/2" End Wrench To Tighten Tru Turn Aluminum Spinner SPECS: Crankshaft Thread Size: 5/16" - 24 Spinner Mounting Bolt Thread Size: 10-32 Bushing Size: Inner: 8mm Outer: 3/8"Tru TurnPart TRU0520A