I was still typing, re read.
As far as size, make an impression on a piece of wood or something. Hit it with a rubber mallet . Stick some playdough on it and take it to the store. Or just mic it, or find a loose one that will come loose with the correct bladed screw driver. Rub a pencil on paper over it or buy several sizes and take some back to store. They are very hard to measure because the sizes are so close and tolerances can almost cause an overlap. Maybe someone with an AD will chime in with the size.
I think the stuff up high, inside a cab should not be rust welded. Hope not. They may all come out clean and easy. I was just warning you in case.
A 100 OF THEM? !!!!!!!!! I told you a million times not to exaggerate.
