As Roger said the thread is 1/4 bsp 19 TPI.
I'm just starting to make 2/3 taps Leader and Arrow, they should be ready in a few weeks time. I make a batch of components and then assemble the taps as needed.
Re modern fuel.
I 've not noticed a significant difference in the life of the corks in the taps due to the fuel, what seems to make the most difference is the quality of the cork ie porous material and harder material. I had some corks a few years ago which were very springy and they lasted a long time.
The corks in my bike taps last from 6 to 10 years with a bit of care. The last one was changed not because it leaked , there was just a suggestion of fuel at the tap end but because it vibrated closed on a long run.
I've experimeted with' corks' made from synthetic wine bottle corks and so far they seem to be doing well. I cut them out using a home made cork cutter.
Regards Brian H.