On my '59 gmc, the original mirror arm had been removed. Some 5/16" screws had just been put in to plug the holes, and those had become frozen with rust. I tried my impact tool to break them free, but it seemed I would either strip the heads or dent the door before that would happen. I gave them a good slosh with WD-40 from the inside after removing the door panel. Next time I was at the hardware store I got a 5/16 rod coupling and another cap screw. I used these to back out the screws from the inside with a flex ratchet and socket. This way I could get enough leverage to break them free without breaking anything else. wink