Hi jlang.
Of the two that I have, one is a 5/16x24 and the other is a 1/2 inch thread. You can measure the diameter and count the threads. Example, 5/16x24 would be 5/16 diameter and have 12 threads in one half inch of lenght. You could alwaye take it to a good hardware store and match it up with the correct nut.

Dave