its a self locking nut! needs to be replaced with new locking nut any time its removed! has a plastitc ring in it the threads cut into to lock it and reusing is not advised and all the loctite in the world ant gonna hold it! went threw it on a 71 duster years ago! when the pinion bottoms out into the carrier at a 7800 rpm 1-2 shift bad things happin!! go by a self locking nut and save the aggravation!
