the pipe work is 3/16 so reckon most likley 3/8 unf
or not sure if we went metric maybe M10 x 1 ?
Chris
Got one for Me , Then one for her, and now a big one for me again, All BLOO! Well saves on the touch up paint, Now Number one son's Spoilt it all by getting a Firefrost 1.8T
The standard pitch for an M10 thread is 1.5mm, but the rarer fine pitches are 1.25mm or 1.0mm.
If you struggle, it may be better to get a proper fitting, and blank it off. I used a short length of pipe, with a flare one end and folded / soldered the other.
oops sorry forget to mention it would be m10 fine pitch .
Got one for Me , Then one for her, and now a big one for me again, All BLOO! Well saves on the touch up paint, Now Number one son's Spoilt it all by getting a Firefrost 1.8T