I believe that the thread for Simplex axles is British Whitworth. I was looking for new axle nuts years ago, but I could not get them to match. My son manages a Fastenal store, and he could not find a match. I called Gary Wollard, and he just fell out laughing. That's when he said the axles and nuts were imported from England. If you find out anything different, I want to know!!!!! Mike