Hi Layden, The diameter is dead right for 1/8 inch American Pipe thread, so thanks for your suggestion. According to my data book the pitch for Am Pipe is 27 tpi. My gauge seems to fit OK at 26 tpi, which is what threw me off the scent, but the diameter is so close that I am sure that you must be right, (and I don't have a 27 tpi gauge). many thanks!! Buick (from the UK)