THREAD IDENTIFICATION

[ray jones 1Participant @rayjones1]

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

[ray jones 1Participant @rayjones1]

UNUSUAL THREAD

[not done it yetParticipant @notdoneityet]

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

[SteviegtrParticipant @steviegtr]

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

Steve.

[EmgeeParticipant @emgee]

Ray

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

Emgee

[Lee RogersParticipant @leerogers95060]

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 FarrParticipant @nicholasfarr14254]

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.

[JasonBModerator @jasonb]

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 FarrParticipant @nicholasfarr14254]

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 TinsleyParticipant @andrewtinsley63637]

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.

Andrew.

[DC31kParticipant @dc31k]

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 LewisParticipant @howardlewis46836]

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.

Howard

[JasonBModerator @jasonb]

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 LewisParticipant @howardlewis46836]

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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