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ray jones 130/07/2021 11:25:50
54 forum posts
29 photos

Hi Everyone

I am repairing a very early English made cast iron fuel tank , age approximately 1910. .

The drain plug is is a brass/ bronze stopper. The problem is I cant identify the thread type/ size.

The stopper plug is 0.424 inches diameter x 25 TPI.

My thread gauge identifies it as 25G

Can anyone identify this please

many thank ray

not done it yet30/07/2021 11:59:03
6346 forum posts
20 photos

Repairing or restoring? I doubt it would be unacceptable if a repair changed the thread to something more ‘standard’?

Steviegtr30/07/2021 12:24:50
2244 forum posts
311 photos

Sorry cannot help. But there will be someone helpful along shortly. Maybe with an actual answer to your question.


Emgee30/07/2021 12:32:52
2158 forum posts
265 photos


I believe you have identified the thread as 25TPI at the given diameter, it may not be a standard thread used today but certainly fits the bill for a brass thread that is now more usually 26TPI


Lee Rogers30/07/2021 12:34:54
146 forum posts

1/2 inch 26 tpi would be BSB ( British Standard Brass ). It's so close that I wonder if another look ar the tpi might be worth a go?

Nicholas Farr30/07/2021 12:45:46
2999 forum posts
1369 photos

Hi, 7 / 16" is close to the diameter that you have measured and the difference between 25 and 26 TPI is hardly noticeable if you are measuring a short section with your thread gauge, so a 7 / 16 x 26 TPI BSB might be OK.

Regards Nick.

JasonB30/07/2021 12:50:28
21451 forum posts
2453 photos
1 articles

7/16 x 26 was also the old CEI (Cycle Engineers Institute) thread so would be a possibility if a 26tpi gauge looks close

Nicholas Farr30/07/2021 13:05:46
2999 forum posts
1369 photos

Hi, as JasonB has said, and the only difference between British Standard Cycle (B.S.Cy.) is 60 degrees and BSB is 55 degrees thread angle, so either of those may work.

Regards Nick.

Andrew Tinsley30/07/2021 16:05:07
1485 forum posts

I have an old book on threads dating from 1914 and the usual Machinery's "Screw Thread Book". The nearest thread to 25 tpi is in fact 0 BA (at 25.4 tpi), but obviously this falls down on diameter and the same for 1mm threads.

I am pretty certain that Nicholas Farr is correct and that it is 7/16" 26 tpi brass thread (you say the plug is bronze or brass!). Tracy tools do a carbon tap for £3 and a die for £4, plus postage, plus Vat, so you could spruce up the threads if need be.


DC31k30/07/2021 17:34:05
578 forum posts
1 photos

If we are going for a 'nearest the bull' on thread angle, diameter and pitch (one dart for each property), can I use my three arrows to hit M11 x 1.0? It is closer on two out of three counts than anything thus far suggested.

Edited By DC31k on 30/07/2021 17:37:21

Howard Lewis30/07/2021 20:27:30
5328 forum posts
13 photos

I very much doubt if a British manufacturer, back in 1910 would use a Metric thread, particularly 11 x 1 mm which is a bit of a fine pitch oddball (even by modern standards )

I would much more inclined to put my money on an undersize Brass 26 tpi thread. Unless it was a "bastard" deliberately made of keep it unique.

We are assuming that the thread form is Whitworth, of course! What is it? (Checking Whit form pitch with a 60 degree gauge will introduce some inaccuracy. )

It could be something REALLY peculiar, that none of us have considered.


JasonB30/07/2021 20:48:44
21451 forum posts
2453 photos
1 articles

I did not assume whit form, I suggested a 60deg form at 26tpi.

I suppose we could also go 1tpi the otherway and come up with 7/16" Admiralty which is 24tpi.

What is the fuel tank from?

Howard Lewis31/07/2021 10:01:20
5328 forum posts
13 photos

In 1910, I would assume, possibly incorrectly, that almost anything of British manufacture would have used Whit form threads

If the plug is the original, thread gauges should settle that argument.

A little more information would make life easier for all concerned.

Howard.  Fat Fingers again

Edited By Howard Lewis on 31/07/2021 10:02:41

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