the nut's missing on it and my pump keeps loosening up but i can't figure out what pitch the stud is cuz the threads are a little messed up, anyone know? i think it's either 3/8 or 5/16 coarse

 

just a WAG but i think it's 3/8ths, i was thinking fine thread but it's been a while since i had one off. just slip an open end wrench on the stud and you'll know what it is by what wrench fits ,then all you have to do is figure out the thread pitch.

 

ede is right about the size, but it's 3/8-16 (std thread)

 

The rear stud on my '86 is an M10 x 1.0, not a fractional size. The rest of the bolts (that thread into the engine case) are fractional. Really convenient, eh?