This is the Drive Washer for use on the O.S. 72FS-a Ringed Engine (OSMG0877). FEATURES: Aluminum construction INCLUDES: One Drive Washer One Woodruff Key REQUIRES: Installation on the engine SPECS: Height: 9.05mm Diameter: 29mm COMMENTS: This part goes on the crankshaft, immediately in front of the crankcase. The prop (or spinner backplate) rests against this piece. cnh 3/10/09 ir/jxs