This is a Flywheel Nut for the OS Max .21XM Out-Board Engine. This Nut Goes Inside the Flywheel (Starter Cone) On the Crankshaft. INCLUDES: One stainless steel 1/4-28 Acorn Nut (Flywheel Nut) COMMENTS: This Nut Will Also fit the .61SF-HS Engine. Nut has reverse threads. ir/jl