I'll let you know very shortly, as i bit the bullet and ordered one from a place over here that sells old stuff. I ordered 1/8 NPTF whatever that means... We'll see when i try and fit it...!!!
If you have a tap and die set, 1/8 National Pipe Thread might be in it,,,,,,,and it kind of looks like a 5/16 bolt size....BUT........it is a tapered thread made to seal as you tighten it,,,,,,,,engine sending units use pipe threads