Soldered union on copper pipe

This topic has 25 replies, 14 voices, and was last updated 2 November 2025 at 23:09 by Bill Phinn.

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1 November 2025 at 23:25 #822728
Roger King 1
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@rogerking1
Sorry, I’m using the ME skill base to ask for advice on old car jobs again…

I’m rebuilding the head on a 1937 Riley, and need to remake the copper pipe feeds to the rocker shafts. These are made of ⅛” OD copper pipe, with a banjo fitting soldered on one end and a brass cone fitting on the other. I’m struggling to find new parts for the cone fitting – see photo below for what’s on the old pipes. Sizes are: copper pipe OD is 0.125″, and brass nut to retain the cone is 0.375″ across flats – I don’t know what the thread size is. The car suppliers all sell a 7/16 (0.4375)” nut and cone to fit ⅛” pipe, which doesn’t work as the nut is too big to thread onto the feed stud from the block. Any suggestions as to what size this nut and cone union are, and how best to do the soldering (ordinary solder, or silver, without melting the components?) would be very welcome.

Thanks,

Roger
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2 November 2025 at 02:10 #822733
Bill Phinn
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@billphinn90025
I can’t help you much with what thread is in the 3/8” af nut and on the corresponding male part (have you got a thread gauge?).

It looks like soft solder was used on that part before you got to it, and the lower joint got badly stressed at some point in time.

I don’t know how much heat the fitted part is subject to. To be on the safe side you might want to use silver solder. If you do use silver solder you’ll need to melt off all the existing soft solder and thoroughly clean the joints first. Alternatively use a new piece of copper pipe.

There’ll be no issues at all with melting the parent metals (copper and brass) unless you go berserk with an oxy-fuel torch and heat things way beyond the solder’s melting point.
2 November 2025 at 06:36 #822737
Speedy Builder5
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@speedybuilder5
https://maccmodels.co.uk/?s=nipples

I appreciate that these come in packs of 10, but note that there are 2 different sizes depending upon the diameter of the thread/ union nut. I think we need more detail of the male thread that the union nut goes on to. Once you know that, I am sure someone here would offer you one. I would, but live in France and postage isn’t worth it.
2 November 2025 at 06:40 #822739
not done it yet
Participant
@notdoneityet
Anneal it and bend it back straight, solder the joint if it leaks. Soft solder will suffice, I expect – the oil temperature will never be above the melting point!

There will clearly be very little pressure involved. Using the original parts, if possible, would be my choice. Is just the one damaged, or all eight?
2 November 2025 at 06:50 #822743
David Senior
Participant
@davidsenior29320
I always just soft-soldered the pieces together (using ordinary plumbers solder and flux) when I was assembling fuel pipes for Morris Minors, which use the same concept but slightly bigger. There should be no difficulty in unsoldering the nipple and re-using on a new pipe.

Dave
2 November 2025 at 09:53 #822765
john fletcher 1
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@johnfletcher1
I’ll go with “not done it yet” above, I have done it several times in the passed on motor bike petrol tanks to the carb. Ted
2 November 2025 at 10:17 #822771
Roger King 1
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@rogerking1
Thanks all, very helpful – I knew this was the place to ask.

A bit more info: the male thread is 0.307″ external thread diameter, and the tpi is 26G.

The temp for the pipe shouldn’t exceed 110-120˚C, by which time other problems will have kicked in…

Pressure in this elderly motor can surprisingly exceed 110psi at the bottom end, but probably not up at the heads.

There are only two of these – one on each side, feeding the rocker shaft, which is hollow. I’ll remake with new parts if possible – the pipes on the old ones have been bent and straightened a lot over the last 80 years, and although annealing would sort that, the pipe and unions are not expensive in the grand scheme of things.
2 November 2025 at 11:00 #822776
Speedy Builder5
Participant
@speedybuilder5
That sounds like a worn 3/8 x 26tpi British Standard Brass thread. Model engineers usually use 3/8 x 32tpi however various manufacturers used 3/8 x 26tpi as they weren’t so prone to cross threading nuts to fittings.

Bob
2 November 2025 at 11:05 #822779
Roger King 1
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@rogerking1
I think the nut might be a special. Someone has suggested it’s British Standard Gas thread, 5/16, which is possible. It needs to be relatively slender to fit under the rocker cover.
2 November 2025 at 11:13 #822780
noel shelley
Participant
@noelshelley55608
0.307 and 26 sounds like 5/16″ BSB brass thread. It would also fit a 3/8″ AF nut. I would un solder the old fittings and re solder on new plpe. It looks as though soft solder has been use – quite common for this sort of thing.

BSB all sizes are 26TPI BUT there is also BSC (cycle ) both are 26TPI but BSB is 55*thread angle and BSC is 60*. on a worn fitting I doubt it will make much difference. Noel.
2 November 2025 at 11:17 #822782
Roger King 1
Participant
@rogerking1
Yes – it’s probably been mucked about with a lot since 1937! The soldering is not the neatest, and it will be a bit of a challenge to clean the parts up.

Thanks all, plenty of help here and I think I’m pointing the right way now.
2 November 2025 at 11:22 #822784
Michael Gilligan
Participant
@michaelgilligan61133
A step on the road to insanity:

https://www.machiningdoctor.com/charts/bsp/#bspp-g-basic-dimensions-chart

MichaelG.
2 November 2025 at 16:10 #822818
old mart
Participant
@oldmart
An old British car may well have had cycle threads in the design, there is a close candidate there while BSB seems to miss out the closest fit. I like to consult the excellent thread charts on the Motalia website.

https://www.motalia.co.uk/
2 November 2025 at 16:45 #822821
Split Pin
Participant
@splitpin
nuts and olives here

https://www.s-v-c.co.uk/categories/stafford-vehicle-components-metal-pipe-fittings-solder-nuts-nipples

No connection other than have used them in the past

Steven
2 November 2025 at 17:47 #822826
noel shelley
Participant
@noelshelley55608
Gas thread sizes have nothing to do with the OD ! eg 1/8″ gas is about .383″ and 1/4″ is .518 OD . There isn’t a 5/16″ gas as standard. The first is 28 TPI and the second 19TPI. Noel.
2 November 2025 at 19:34 #822837
Nimble
Participant
@nimble
Hi Roger,

Surely there are many Riley Clubs in the UK or elsewhere that would be able to give you info on the unions needed for your oil line. These probably have Forums that can answer your query.

If you have trouble finding this info, post the fact that you can’t get it and I will enquire from a friend with a RM Riley if he has any info re oil pipe lines.

Nimble Neil.
2 November 2025 at 19:59 #822846
Roger King 1
Participant
@rogerking1
Thanks Neil – yes, I’ve asked the Riley Register (of which I’m a member), and received some definite, yet contradictory responses. Also Riley, as part of the great British Motor Industry, were inconsistent in their design continuity, which has something to do with their going into receivership the day after my other Riley was built in 1938. What fits one model doesn’t necessarily fit another… The RM series were all post-war, and built under Nuffield ownership, pre-BMC.

Thanks for the kind offer, but I think I should be sorted now.
2 November 2025 at 20:17 #822848
Charles Lamont
Participant
@charleslamont71117
On 2 November 2025 at 17:47 noel shelley Said:

Gas thread sizes have nothing to do with the OD ! eg 1/8″ gas is about .383″ and 1/4″ is .518 OD . There isn’t a 5/16″ gas as standard. The first is 28 TPI and the second 19TPI. Noel.

Surely that is BSP, not gas thread?
2 November 2025 at 20:56 #822855
Andrew Crow
Participant
@andrewcrow91475
In the dim and distant past in many industrial environments BSP was often referred to as gas thread.

Andy
2 November 2025 at 21:25 #822869
Nicholas Farr
Participant
@nicholasfarr14254
Hi, Gas and Pipe threads are the same, and as Andy has said, they used to be called Gas many years ago. As far as I know, there are no standard pipe threads with 26 TPI, and there are no British pipe threads that have any 16 th’s in their size.

Regards Nick.
2 November 2025 at 21:29 #822873
Roger King 1
Participant
@rogerking1
As mentioned earlier, I’m pretty sure these will be either BSB or BSC – pre war cars frequently use both.
2 November 2025 at 22:18 #822881
Charles Lamont
Participant
@charleslamont71117
So, what is the correct name for the constant-pitch 26TPI Whitworth-form brass pipe thread? As, apparently, no British Standard exists for it, calling it British Standard Brass is evidently BS.
2 November 2025 at 22:25 #822887
Roger King 1
Participant
@rogerking1
Is it not BSC – British Standard Cycle thread, BS811?

https://en.wikipedia.org/wiki/British_Standard_Cycle
2 November 2025 at 22:32 #822890
noel shelley
Participant
@noelshelley55608
I’m not aware of any 26TPI pipe thread ! 40TPI is often used on copper pipe but that does NOT make it a pipe thread. Nuts and fittings were made of brass so why not BSB ? Noel
2 November 2025 at 22:49 #822893
Andrew Crow
Participant
@andrewcrow91475
Several of my 26 tpi taps are actually marked as BSB or just Brass.

Andy.
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