You may have to change to 1/8 BSP, which is the closest, with easy availability of pipe and fittings. There is a Model Engineers thread of 3/8 x 32, not big enough for your fittings.

Royal Enfield used European engines early on so probably metric.

This site may be helpful Thread Data

I have I hope sent the link to metric pipe threads but there is a great deal of information on this site relating to threads

edit, I don't see exactly what you are looking for but you can search further !  Worth remembering that "standards" were variable 100+ years ago many used their own standards and often used odd ball sizes so you had to buy parts from them !

John

Edited By JohnF on 22/10/2022 10:23:57

I work on early vintage cars and this is quite common. Bugatti are know for using odd threads even bsp threads on their gear boxes!

Checking the thread form will give an indication of the possible thread.

A Metric thread should be 60 degrees, while an Imperial, BSW, BSF,, BSB, or BSP will be 55 degrees.

Cycle threads are 26 tpi, but 60 degree form.

Not that it will be BSP,, since 1/8 BSP is 0.385 OD but 28 tpi.

Not BSB since that is 26 topi for all sizes

The only 32 tpi BSF is 1/8 which is too small a diameter.

If it is not a "Special", as said it might be a slightly undersize 10 x 0.8 metric, since 0.8 mm pitch is less than a thou different from 32 tpi .

Edited By Howard Lewis on 24/10/2022 05:51:31

Interesting time-line here: **LINK**

https://www.royalenfield.com/uk/en/our-world/since-1901/

Noting that the 1909 Royal Enfield motorcycles used a Swiss-made engine.

… May, or may not, be relevant to the mystery thread

MichaelG.

.

Wow! __ https://www.motosacoche.swiss/en

Edited By Michael Gilligan on 24/10/2022 09:41:08

That’s quite something, Keith

Thanks for sharing the photo.

MichaelG.

I know I am late to the party, but could it be that they are metric cut on an imperial lathe but without the benefit of a 127 tooth gear. Some of the approximations are quite a bit off.

Or use a hand-crank on the lathe spindle to make things more controllable. Or make your own tap out of silver steel.

If you want some very accurate change gear combinations to cut metric threads on an imperial lathe, see the Workshop Practice Series book Screwcutting in the Lathe by Martin Cleeve. Accurate withing a thou in 8 inches etc, using standard changegears set up in compound trains. He lists the full chart.

For .8mm pitch using a Myford 8tpi leadscrew, he quotes an error of 1 thou in 1.144 inches. That is using a gear train of 20 — Any Idler — 55/45 — 65.

Obviously, 20 is the headstock spindle gear (or reversing tumbler output gear in Myford case) and 65 is the leadscrew gear. And the 55/45 are on one stud with the 45 meshing with the 65.

BTW, that is one beautiful old bike. Phenomenally well preserved and complete for that vintage. WOW! Will you preserve that original paint and patina or hide it under a layer of modern two-pack paint? The glass oil tank is unique. Maybe a step up from a glass drip oiler on a steam engine?

Edited By Hopper on 25/10/2022 07:32:19