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How to identify an Imperial thread size?


blackrose

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I have an old Simpson-Lawrence anchor windlass and I need to replace the nut which holds on the gypsy assembly.

 

The measurements are approx 735/1000" ID and 1298/1000"AF

 

How can I tell what the thread is or what it's likely to be? Does it look like BSF or UNC?

IMG_20200907_190633_737~2.jpg

Edited by blackrose
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36 minutes ago, David Mack said:

Can you measure (approximately) the outside diameter of the thread and the thread pitch?

 

Thread is 847/1000" OD (approx), so thread depth is about 112/1000".

I've no idea how to measure the thread pitch but the nut is 508/1000" thick and has 4 threads (I think!) 

Edited by blackrose
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22 minutes ago, blackrose said:

847/1000" OD (approx).

I've no idea how to measure the thread pitch but the nut is 508/1000" thick and has 4 threads (I think!) 

847/1000 is close to a worn 7/8"  (0.875). You need to use a thread gauge to measure the number of pitches per inch, that can then be checked on a thread chart to establish what the thread is. If it is a BSF or BSW thread I may be able to find a suitable nut.

 

Edited to add:- 7/8"BSW has 9 tpi, whilst 7/8" BSF has 11tpi

 

 

 

Edited by David Schweizer
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Go to an engineers supllier or a fastening specialist and they will check it for you and with luck be able to supply a replacement. If you are near Loughborough I would recomend Charnwood Fasteners. I've been in there with an odd sized bolt and they've just looked at it and said what size it was and what thread. They then found one for me.

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20 minutes ago, David Schweizer said:

847/1000 is close to 7/8"  (0.875). You need to use a thread gauge to measure the number of pitches per inch, that can then be checked on a thread chart to establish what the thread is. If it is a BSF or BSW thread I may be able to find a suitable nut.

 

 

 

Well it would be approx 8 threads per inch.

13 minutes ago, Richard T said:

Go to an engineers supllier or a fastening specialist and they will check it for you and with luck be able to supply a replacement. If you are near Loughborough I would recomend Charnwood Fasteners. I've been in there with an odd sized bolt and they've just looked at it and said what size it was and what thread. They then found one for me.

I'll try to find somewhere near Northampton.

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18 minutes ago, Tracy D'arth said:

Is this English equipment, and very old? If so it could be Whitworth.

If its American it won't be, more likely UNC

If its continental then metric but its not a metric AF  size.

Simpson Lawrence were based in Glasgow, I believe that they ceased manufacturing windlasses and boat equipment about 15 years ago.

 

 

Edited by David Schweizer
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16 minutes ago, David Schweizer said:

Simpson Lawrence were based in Glasgow, I believe that they ceased manufacturing windlasses and boat equipment about 15 years ago.

 

 

Its unlikely that they were building with Whitworth threads 15 years ago but I suppose that they could have retained some very old tooling, Most Whitworth stuff disappeared after  WW2 when the American lease lend equipment came over with Unified threads. Most cars after the war were all unified threads.

 

Logically I would have said UNC, its definitely a coarse thread but 1.298" does not equate to a unified AF nut size.  1 1/4" AF or 1 3/8" AF  would be expected.  Both are unusual sizes though. Similarly 33mm is very uncommon in the metric range.

 

Matching at an engineers' suppliers would be the best course of action.

Edited by Tracy D'arth
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4 minutes ago, Stilllearning said:

You need a thread gauge

 

https://en.m.wikipedia.org/wiki/Thread_pitch_gauge

 

 

Whilst I think about it, it’s worth looking into the work of Mr Whitworth, a fascinating man who I vented so much useful stuff.

And don't forget Armstrong as well, real engineers.

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1 hour ago, Tracy D'arth said:

Its unlikely that they were building with Whitworth threads 15 years ago but I suppose that they could have retained some very old tooling, Most Whitworth stuff disappeared after  WW2 when the American lease lend equipment came over with Unified threads. Most cars after the war were all unified threads.

 

Logically I would have said UNC, its definitely a coarse thread but 1.298" does not equate to a unified AF nut size.  1 1/4" AF or 1 3/8" AF  would be expected.  Both are unusual sizes though. Similarly 33mm is very uncommon in the metric range.

 

Matching at an engineers' suppliers would be the best course of action.

Blackrose does not say how old the windlass is, except that it is "old", but Simpson Lawrence made them for about a hundred years so, depending upon it's age, there is a good  chance that the thread could be Whitworth, which was still around for a lot longer than the 1940's. I have no recollection of seeing anything with a UNC thread until the 1960's. Most of my early cars had both BSF and Whitworth threads, the only modern feature being that some of them had AF heads, but I still used my BSW/BSF spanners a lot more than the AF ones.

 

 

Edited by David Schweizer
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Chewing gum, pressed into the nut something less than half the diameter, if possible chill and then carefully remove. Inspect carefully with a good steel ruler and magnyfing glass. Do it more than once to be sure although I would not recommending using the same piece of gum.

Knowing the pitch is usually the clincher.

 

Flthy habit and about the only thing the stuff is good for.

  • Greenie 1
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30 minutes ago, Machpoint005 said:

All of the above aptly demonstrates what a wonderful idea metrication is! I realise this is no help to anyone with a penchant for vintage stuff.

Standardisation is good, but it did not need to be metric. Both BS and UN threads could have been adopted, and as the give an option for both fine and coarse threads depending on the use.

I see that on continental plumbing and pipework equipment, we still rely on good old BSP, but again there's a choice of parallel or taper, but we gave up on compression.

My guess on that nut is  7/8 BSW. 

Edited by Ex Brummie
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9 hours ago, David Schweizer said:

Blackrose does not say how old the windlass is, except that it is "old", but Simpson Lawrence made them for about a hundred years so, depending upon it's age, there is a good  chance that the thread could be Whitworth, which was still around for a lot longer than the 1940's. I have no recollection of seeing anything with a UNC thread until the 1960's. Most of my early cars had both BSF and Whitworth threads, the only modern feature being that some of them had AF heads, but I still used my BSW/BSF spanners a lot more than the AF ones.

 

 

Wasnt UNF & UNC known as American fine (ANF) and American course (ANC) 

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No.  The are several points of difference between ANC and UNC and ANF and UNF.

Some are quite technical, but important to proper function under load,  like the detail of the thread form, but not all the pitches are identical for a given diameter.

N

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