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

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.

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.

On 13 April 2025 at 09:30 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