Well, the dealership had a part #, but it was discontinued. It's 9/16-18 thread. I ended up going to GM sports salvage and pulling the nut off a 64 GTO pump. I don't know if you have any places like that in Texas, but GM Sports Salvage has everything from old Gm's to new ones. You're going to have to find something older than 1975 though. The newer pumps have pressed on pulleys.