On my Howse, I think the stumpjumper is on a tapered, splined shaft. I don't know how the center nut could work loose if the cotter pin is still in place and intact, unless the splines have worn somehow or something. Anyway, the center nut on mine was loose too, so I took it off and cleaned and greased the splines and threads before I put it back good and tight, with a new cotter key, hasn't come loose since.............