Use a lock nut as a jam nut,the one with the plastic in it.Or,install a castle nut,then drill a hole [small]through crankshaft and through a space in the castle nut and use a cotter pin.If ya think the cotter pin might wear and let go,try a steel nail that fits snug and then bend over and snip off the excess.Dont worry about anything being off balance,your to close to the center of crankshaft to matter.