hi john, need a little clarification, nuts screw onto shafts, bolts screw into shafts, the standard thread on briggs drive shafts is 3/8 unf, if it really is a nut you need most towns have stores that specialise in fixings, belts, bearings, oilseals etc remove the engine (3 bolts) and take it into them,they will identify the thread, get a self locking nut. hope this is of some help
tony daly, May 2014
