Not positive, but I think they are 1/4 x 28, or they could be 5/16 fine thread.
I saw your post a few days ago but have been busy, I did look for one. I haven't been able to locate one to check it. I probably have 20 of them in a coffee can somewhere, problem is locating the correct coffee can. I will try and look again today. In the mean time keep your pants on!
