I'm replacing the impeller in my 84' Alpha drive's lower unit. All the lube is out and thankfully looked great, but I'm having an issue with removing two nuts. All of the other nuts and bolts came free no problem, but the two bolts under the cavitation plate right above the prop are of odd size. I tried 9/16, too small, 5/8 is too big to go into the recessed area where the nuts are, I tried 14mm and that's too small but 15mm is not working either. Is there a special socket for these? I've never ran into anything like this working on my previous Alpha drives. Any input is welcome.