Mystery ancient thread size

I have a couple of Royal Enfield motorcycles, each in excess of 100 years old. In the course of the restorations (running concurrently), I need to make some compression fittings male and female for oil circulation lines. The thread count of the existing fittings is definitely 32 tpi, but the diameter is odd at 0.392" over the crest of the threads - suspiciously close to 10mm.

Can anyone help as to whether this is from a recognised thread form/classification that I am unaware of, or maybe they're just a special fine thread that the manufacturers came up with. This is the only size/diameter other than standard Cycle thread in use on the bikes.

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.

10mm x 1.25mm pitch is very close - maybe its that?

Royal Enfield used European engines early on so probably metric.

1.25 pitch is nowhere near 32 tpi

try 0.8mm pitch

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

Oops! Did my sums the wrong way round - meant 0.8mm - thanks Ian

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!

We would be interested to know more about the bikes, and what models they are.

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

I know nothing about motor bikes, but I'm sure I read somewhere about one maker using daft threads like 17/64"BSW so that you had to get the spare parts from them. So check very carefully!

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

Thanks for all the responses folks - I'm beginning to think the thread is 10mm x 0.8 pitch, having tried a metric pitch guage now as well as an imperial one, as I can't see the difference over about 10 threads.

Frustratingly, I could retain all the original fittings I have, which are double ended, one end threading into the crankcase/oil tank/oil pump etc, the other end to a compression fitting of a missing oil line. However all the fittings I need are the compression nuts, so I'd have to screwcut these threads internally into a blind hole - not my favourite pastime!

I may remake the double ended male components 10mm x 0.8 pitch one side, and 26 tpi cycle the other, and make all the compression nuts 26 Cycle also. Would be happier using a tap up the nuts.

I specifically didn't include much motorbike detail as I thought the post may get moved, and the thread i/d was the important issue, however if I can remember how to attach a photo I'll put one up. I do have an RE with the Motosacoche engine (1912), but the bike in question is from 1915 and has Enfield's own in house engine with a totally different oiling system to the Swiss engined bike (dry sump recirc, rather than total loss - way ahead of it's time). Notice the GLASS oil tank.

Edited By keith hodgson on 24/10/2022 16:25:46

Edited By keith hodgson on 24/10/2022 16:26:36

That’s quite something, Keith

Thanks for sharing the photo.

MichaelG.

Keith, Nice looking project you have there; by coincidence, I was watching a video last night on thread milling in the lathe.




The other possibility, is to run the lathe backwards and cut away from the headstock.



Bill

Edited By peak4 on 24/10/2022 20:46:27

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.

Keith, don't be put off cutting internal threads, you can start inside the nut and screw away form the chuck so no chance of a crash. I know if its a screw fitting chuck it could unscrew but it is IMO extremely unlikely with such small dia & fine pitch thread -- I have cut many many threads this way on my Myford without any trouble. Plus its possible the material will be brass ?? so even less cutting force.

Use a bed stop and make a small undercut at the start point, assuming your lathe is imperial for ease I would consider using 32 tpi so you can use the chasing dial. Even if you felt the need to cut metric pics on an imperial lathe you can still use the change dial but not in the conventional manner. If you need details for this procedure please message me.

The pitch difference is only 0/00025" per turn so 0.0025" over 10 threads so a tiny movement of the top slide on the last cut will accommodate this. Depending on the accuracy/sizing of the original; parts you may not even need this -- try one and see how it fits.

John

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

Nigel

One of the culprits to use odd size threads on their motorcycles was Douglas, on their Dragonfly model, only obtainable from the makers. Fine until they went out of business, this led to some horrendous bodges by amateur mechanics in the 60's and 70's. I was given a dreadful example in the mid 70's and spent hours sorting it out only to find it a most disappointing thing (as a nutty 19 year old) to ride!

Dave