Starter Motor nut removal

“93 Mercury Sable. Trying to get the old starter motor off to put another one on. I’m having trouble removing a nut. It looks like it’s tapered. The 1/2” and 13 mm sockets go about half way down and won’t fit snug. The 5/8" and 14 mm sockets go all the way down but are too loose. Like I said unless I’m wrong this thing is tapered and therefore nothing fits snug around it. Am I wrong or are engineers sometimes stupid? Or both. Any help would be appreciated Thanks