To Cap It All

Hello Everyone

I wonder if I might pose a couple of questions and try out an idea on the experts?

I want to make a bronze vintage radiator cap (I do know I could buy one but they seem disproportionately expensive and I like to make things!)

I thought... if I bought a nut of the same thread as that on the neck of the header tank, I could turn off the hex on the lathe, mill a bronze replica of the original cap and solder or braze it to the nut. Simples! Job done.

However, I can't for the life of me, and after consulting every table I can find, determine what the male thread is. The major diameter is 37mm 16 TPI. Any ideas?

Now I acknowledge the proper way to do this job is start with raw bronze and cut the female thread on the lathe, but I don't feel I have the competence or confidence to do this, not least because there are a few fundamentals I'm unsure of.

I use a Chester Super B lathe configured for metric. Assuming I know the lead screw pitch, can I gear a metric lathe to cut imperial threads, as would be required (I guess) for the radiator cap. And if I can, and someone knows the thread I'm talking about, what change wheels might I be looking at.

Hope all this is not too bizarre.

Best, as always, from la belle France.

Martyn

Addendum!

As regards the thread - I've seen the major diameter expressed as 1.475 inches, which is about 37.4mm isn't it?

Martyn

My trusty ‘Fractions Calculator’ gets 37 and 93/200 mm [37.465] … but, whichever way you express it, I don’t recognise that as a thread size for which you could expect to buy a standard nut.

MichaelG.

A 1" brass commpression fitting might be worth a try - the nut ! Or the brass fittings used on plastc pipe may be big enough. I will see if I have one and can measure the thread ! Some had fine threads others much coarser. Talk to a plumber, or look in his scrap box. Good luck Noel

Vintage radiator threads are, in my experience, all over the place, but a Model A Ford is about 16tpi ( much larger od though). This Ford thread would certainly be 60 degree form. Vintage Vauxhall is much coarser at 8tpi.

Martyn

According to Andy Pugh if its 37 mm diameter its a PG29 conduit thread to DIN 40430.

Andys list gives :-

Designation Pg29
Type COND
Major Diam imp 1.457
Major Diam mm 37.00
TPI 16.0
Pitch mm 1.588
Core Diam inch 1.3969
Core Diam mm 35.480
Thread Depth inch 0.0299
Thread Depth mm 0.760
Thread Angle mm 80°

Seems awfully shallow for a radiator cap thread but the numbers fit.

That said 16 TPI is a permitted constant pitch thread under the Whitworth system but you'd expect the diameter to be 1.5 inch if it were Whitworth. Constant pitch size nuts in Whitworth don't exist. Specials only.

Clive

PS Rattus Crappitus. Spent ages sorting that into a nice list with extra spaces and the stupid forum software just stripped them out.

Edited By Clive Foster on 28/08/2021 10:11:16

It's quite possible it is 1.5" nominal with worn or over rounded crests and could be 55 or 60deg

Though the originally quoted 37mm x 16 would make it a PG conduit thread but usually only thin pressed steel nuts available for that

Beaten by clive, just

Edited By JasonB on 28/08/2021 10:10:20

I have a couple of larger radiator caps (c49mm OD) both with 16 TPI threads and deduce that this may have been the standard for this kind of thing.

I assume that your smaller cap is for an Austin Seven. My own 1931 machine came with a presumably non-standard one with built-in temperature gauge; after this was stolen I replaced it with one that resembled one of the couple:

Not an answer to your question I'm afraid but I thought this might be of some interest. In your position I would regard the machining of the body with its pleasing form to be the difficult bit; the screwcutting looks straightforward.

What vehicle radiator ?? British / American will use TPI and most others will be some form of metric generally.

37mm is about Austin 7 size. which I believe is 1 1/2" and the thread would be Whitworth form and any TPI the maker chose.

Bob (1932 Austin 7)

What make of vehicle is it ?

I’m with Jason it’s a worn 1 1/2” x 16, I have done one in the past.

Probably made undersize rather than worn.

Radiator cap threads are usually made fairy slack to help dissipate pressure during unscrewing if the cooling system is still hot rather than blasting it off as it come to the end of the last thread.

The extra 25 thou clearance by making the majot diameter 1.475 inches rather than the "proper" 1.5 inches for a constant pitch Whitworth thread is entirely reasonable.

Clive

and 1929 Chummy at 1 1/4" by 13tpi

Mike

Many, many thanks everyone. An education in itself!

I'll pursue PG29; maybe ask our plumber, who, resident on the Somme, may not often deal with Austin Seven radiator caps,

Thanks again

Martyn

You would do better asking an electrician than a plumber as it's a conduit thread. But as others have said 1.5" x 16 most likely on an A7

Why not try ringing a radiator refurbished/maker, I have one near to me and there was one recently on a car rebuild program recently.

Martyn,

You have said that " if I bought a nut of the same thread as that on the neck of the header tank, I could turn off the hex on the lathe, mill a bronze replica of the original cap and solder or braze it to the nut"

Therefore you must have access to a lathe and mill so why not make a cap yourself. Not too difficult and will be a good learning curve for some new skills. Try plastic or aluminium for a trial first, then make one out of brass or bronze, A good feeling after you have mastered it.

Paul.

Edited By Paul Lousick on 28/08/2021 14:11:42

I would be inclined just to buy one new. I've had good service from the Seven Workshop in the past. Probably cheaper than a chunk of bronze to make one.

Russell

We have them as well - electricians!

I know...key in your credit card co-ordinates and you have one by return (or something) of post from the usual suspects. Or most other Austin Seven parts for that matter..

I just wanted to make one, which is what I thought we were about. And I wanted to learn.

Best

.

That’’s the spirit

Unfortunately; if the original is like ega showed … I would think it's formed from tube, and then closed with a soldered-on cap.

I would love to see how they actually made that.

MichaelG.