It would seem though that the lower fitting has a 60° female taper as per BSP....so if it was unique to Enots then it followed BSP conventions.
Some careful examination is called for to check if it has a 55° thread angle. If so then it is likely that Benton & Stone created their own intermediate 3/16" BSP size....
For my breather pipe, which thanks to Ian we’ve established is BSC Coarse 7/16 diameter 20tpi. I only managed to source a 7/16 diameter 19tpi fuel pipe union (the ones that fit the Enots fuel taps, also discussed in this thread).
Using a 1.25 pitch metric thread file (60 degree thread form like BSC, much cheaper than a BSC thread file and has got me out of trouble with lots of BSC thread issues) I’ve ‘eased’ the thread in the union. So it now screws onto the breather with slightly more effort than my sausage fingers can produce. I’m happy with this compromise. Once my workshop is built and I have bought a lathe, such things will be a thing of the past!
