Pipe Unions, Fittings, Standard Dimensions, CAD

[MEinThailandParticipant @meinthailand]

Are there any published standard dimensions for pipe fittings as used by model engineers? Things like pipe unions, union nuts, nipples, olives with their appropriate pipe diameters and thread sizes. Also, various companies offering such fittings have different threads for the same size fittings, for example 1/4 X 32, 1/4 X 40 TPI – is there any preference or standardisation here?  Any data would be much appreciated and I’m dreaming, I know, but CAD drawings would also be terrific.

[David George 1Participant @davidgeorge1]

RS components have data sheets with all the dimensions IE Enots fittings

https://docs.rs-online.com/a938/0900766b810f3410.pdf

If you go to the Enots web site you can search for parts ie olives and select and download cad drawings.

norgren-embedded.partcommunity.com/3d-cad-models/sso/360501-universal-tubing-sleeve-suitable-for-nylon-metallic-tubing-imi-norgren?info=imi_precision%2F3_fittings%2F36_series%2Ftubing_nuts_sleeves%2F36_0501.prj&cwid=2897

Other manufacturers may have the same options.

David

 

[John HinkleyParticipant @johnhinkley26699]

Have a Browse on the Traceparts site for “Pneumatic fittings”  There you will find all manner of Imperial fittings in the sort of sizes that you’re after, in more CAD formats than you can shake a very big stick at.  You may have to register to download, but the resources are enormous and worth it, in my opinion.  (It’s free to register.)

John

 

[MEinThailand]

On 12 April 2025 at 02:20 MEinThailand Said:

Are there any published standard dimensions for pipe fittings as used by model engineers? Things like pipe unions, union nuts, nipples, olives with their appropriate pipe diameters and thread sizes.

For fitting used on models (as opposed to around the workshop), I am not aware of any standard. Too many variables.

[JasonBModerator @jasonb]

As Charles says they do vary a bit between makers od ME fittings. I doubt you will find CAD files of these type model engineering fittings

There was an article in ME a year or so ago that had a table of sizes if you wanted to make your own nuts and nipples for each of the common copper pipe sizes. The only thing you really need to watch is if connecting to a commercial fitting is the TPI, otherwise the rest of the dimensions work OK. Mainland Europe would mean a change to metric fine and metric hex sizes a sthat is what the suppliers there such as Knupfer sell.

[noel shelleyParticipant @noelshelley55608]

Whether the ME threads can be called standard is open to debate, and which one to use for a given job. The 2 common standard pipe threads that are small enough are 1/8″ and 1/4″ BSP but there are the 26TPI, 32TPI and 40TPI thread series,or metric. Don’t get Cycle 26tpi and Brass 26tpi mixed up there not QUITE the same, 60* and 55* thread angles. There is some info on metric threads for use in model engineering in the metal workers data book that might be helpful. If working to a drawing then the thread to use is usually specified. Best to write down little gems as you find them in a book and refer to it as need be. Good Luck. Noel.

[MEinThailandParticipant @meinthailand]

Thank you everyone for your thoughtful and useful replies, I much appreciate it.

One thing above all else stood out for me and it was the suggestion by Noel “If working to a drawing then the thread to use is usually specified. Best to write down little gems as you find them in a book and refer to it as need be”, because this summarises the situation.

Everyone (model engineers and suppliers) seem to be doing their ‘own thing’ with no common standardisation.

I’m building a 3/4″ scale traction engine to a design published in 1950 by Bassett Lowke. (for sentimental reasons – I want to bring my childhood memories back to life).

But there a no dimensions of boiler fittings or threads on the 1950 drawings. So what size do I make the safety valve thread for example? 5/16 X 32, 1/4 X 32 or 1/4 X 40, or whatever else?

I’ll be buying a commercial valve and pipe unions so I don’t need to worry about the size of the hex on the unions, but if I was making my own fittings then I’d need to know this. But the info just isn’t there.

David George1 suggest a couple of sites where dimensions for pipe fittings are available but again these are for full sized fittings not for model fittings.

John Hinkley kindly mentions Traceparts, to which I have subscribed but again the parts are not model engineering sized parts.

Table of Pipe Union Nuts Dimensions

I did find a useful Pipe Union Nuts Dimensions on this forum at https://www.model-engineer.co.uk/forums/topic/what-hexagon-sizes-for-pipe-fittings/#post-595032

Posted by Michael Gilligan
Dated: 19 April 2022

The Table is in the form of a graphic, 906946.jpg

I can see that this graphic has since been translated into a typed Table that can be downloaded from:-
https://modelengineeringinthailand.com/forum/index.php?threads/pipe-union-nuts-dimensions.56/

There is also a lively discussion amplifying this top there.

Love to hear more feedback and comments on this subject. Many thanks.

[MEinThailand]

On MEinThailand Said:

…Everyone (model engineers and suppliers) seem to be doing their ‘own thing’ with no common standardisation.

I’d describe Model Engineering (whatever that is!) as being “lightly standardised”.   Though ME’s don’t maliciously go for non-standard fittings, nor, with some exceptions, do they have particular rules.

…

But there a no dimensions of boiler fittings or threads on the 1950 drawings. So what size do I make the safety valve thread for example? 5/16 X 32, 1/4 X 32 or 1/4 X 40, or whatever else?

If a commercial safety valve is bought, then the boiler is tapped to accept it.  If an existing boiler is to be fitted with a replacement safety valve, then the thread has to be identified, and a safety valve bought in that size or made to fit.  The ME has to sort it out!

I’ll be buying a commercial valve and pipe unions so I don’t need to worry about the size of the hex on the unions, but if I was making my own fittings then I’d need to know this. But the info just isn’t there.

Perhaps because it doesn’t need to be!  Model Engineering rarely applies “American Manufacturing” techniques, i.e. production methods depending on accurate measurement such that all parts are interchangeably made to close tolerances.  We use an older technique, in which parts are made to fit with other parts in one mechanism, and aren’t necessarily interchangeable with the same part on another example of the model.

A Model Engineer might choose between ⁵⁄₁₆” X 32, ¼” X 32 or ¼” X 40 simply because he has a tap and die for one and not the others!   Or maybe to keep costs down he’s standardised his workshop on Whitworth rather than BA, ME or Unified.  Or he could be fully equipped to do BSC and BSB as well.

May be best to go with what’s available locally.  In Thailand, US and metric threads may be cheaper and easier to source than ye olde British?  In the UK I prefer metric because for historic reasons there are far too many Imperial Threads, mostly obsolete or obsolescent in 2025.  But then I don’t build 1950 Bassett Lowke traction engines from from old plans! Unfortunately no chance of standardising what went on in the 19th and 20th centuries!

Modern ME designs tend to be metric, which is less confusing when building new.  Metric doesn’t help with older stuff because converting Imperial to metric, or vice versa, is error prone.

Choosing between threads, textbook reasons might help.  Coarse threads are good for soft materials like cast-iron, the thread form is robust, and they can be fitted and undone faster; harder to cross-thread a coarse thread than a fine one! Fine threads are stronger and seal better, so theoretically a better choice for a safety valve, the cost being they are a bit fiddly to make and fit.    As Model steam engines aren’t highly stressed, I think coarse or fine would do equally well.

I recommend Tubal Cain’s “Model Engineer’s Handbook” .  Other vendors available!

Dave

[TrickyParticipant @tricky]

The boiler fittings specified for this TE were listed in the Basset-Lowke catalogue and were sold separately.  These are taken from my catalogue – circa 1960!

The safety valve was 5/16″ x26tpi

The water gauge 3/16″ x 40tpi

The blower cock 5/16″ x 40tpi

The check valve 3/16″ x 40tpi

The pressure gauge syphon 3/16″ x 40tpi

Do you have the booklet called “How to build a Traction Engine” that refers to the 3/4″ scale TE and is very helpful?

If not, I can scan mine in for you.

Richard

[MEinThailandParticipant @meinthailand]

Hi Tricky and thanks for that info. I never thought of looking in my own Bassett Lowke catalogue so I just did that and did indeed find a Table of Threads for Boiler Fittings. on page 188:-

But this catalogue is dated 1932! So the above table isn’t of much help, really.

I’d be very interested to see the page(s) from your 1960 catalogue, though if you could manage it, Tricky.

And thanks for the offer of the booklet called “How to build a Traction Engine”,  that would be very useful, too. I do have ‘instructions’ comprising 12 pages plus 2 pages of appendix but mine’s called “A WORKING MODEL OF A ROAD TRACTOR – How to make a realistic model from Standard Castings”. I wonder if yours is different – the title certainly is.

About the sizes you give, the one “The pressure gauge syphon 3/16″ x 40tpi” is very small. I couldn’t find a supplier offering such a small size.

Thanks again for such great info.

 

[TrickyParticipant @tricky]

Hi MEinThailand,  the booklet I have is the same as yours but with the addition of 4 pages.  I have put these, together with the relevant pages from the Basset-Lowke Catalogue, in a pdf in my gallery.

The commercial pressure gauges seem to have a 1/16″ pipe fitting so the syphon can easily be made from some 1/16″ copper pipe and a suitable adapter to screw into a bush of your choosing. However, I did find Polly Model Engineering sell a 3/16″ x 40 syphon.

Hope this helps.

Richard

[MEinThailandParticipant @meinthailand]

Tricky that’s wonderful information and I thank you so much. I’m having a problem finding the pdf in your Gallery – doesn’t seem to be anything there. Ive tried Galleries, My Gallery and Documents.

Grateful if you could help me out.

Thank you.

[TrickyParticipant @tricky]

Sorry, the upload got put into a gallery that was already there and so was hidden.  I have now created a new document gallery with the pdf in it.

[MEinThailandParticipant @meinthailand]

Wow! Tricky, I downloaded the document you posted and it’s terrific. Extremely useful not only does it give thread sizes and TPI for the different fittings but it also gives details of some fittings specially for the 3/4″ Bassett Lowke model I’m building – the displacement lubricator for example.

This is so useful to me.

Not only that but it gives prices too. For example the big displacement lubricator is priced at 14/7 which I take to be 14 shillings 7 pence. I just checked that with MS Copilot and that turns out to be £32 in 2025. This is the price of the same lubricator today from MaccModels. Fascinating.

I can’t thank you enough for helping me and giving up your time to help me with my query.

 

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