Are there 'Standard' thread sizes for model engines?

[razzle]

I am sure that this question gets asked repeatedly, but I am asking again. I am about to embark on my first small steam engine build from a set of castings i got on ebay. The 'Little Oscillator' found here: Steam Engine Price List | Precision Service I know that plans usually list out the fasteners needed for that engine, but I need to start purchasing taps and dies to make fasteners or start stocking up on fasteners to have on hand. Being in the US it can get tricky. Do I use UNC, Metric, or BA? The choices have large ranges of pricing for quality. Is there a list somewhere that shows the standard sizes for model engineering in each system? Is there any benefit of one over the other? Am i making this too complicated? Thanks for the input.

 

[metalmangler]

It depends... - try emailing the vendor and asking what threads are used for that kit.

 

[rklopp]

I am a fairly advanced modeler and hate BA. I have not used it in 25 years. I mostly use UNF and metric. UNF is closer to scale. Metric is between UNC and UNF in scale pitch. UNC is too coarse and small UNC taps are fragile. I often mix and match UNF and metric to get the best scaling, and often make my own screws and nuts. I like to use form ("roll") taps where possible. Good luck finding BA form taps in the US and good luck single-pointing BA threads. There is nothing wrong with using threads other than those on the print when building a model for yourself.

 

[Brian Rupnow]

I've built a few models---Strangely enough, #5-40 imperial is one of my favorites for model building. It's not a popular size, but it just seems to work so well scale-wise. Also #4-40, #8-32 and #10-24. I use very few #6 shcs, and seldom get up to 1/4"-20.---Brian

 

[razzle]

I forgot there is a unf. I have seen 5-40 mentioned but it seems to be a bit harder to find the taps and dies. I like the idea of staying in imperial. I am very familiar with it so I can identify it quicker. And the metric stuff on my 3d printers drives me nuts. I never have the correct length and the big box stores never have the size I need when I need it.
I already have cheap taps and dies in #6, #8, and #10. So I just need to buy some quality #4-40 and #5-40. Thanks.

 

[rklopp]

I agree with Brian about #5-40. It's 1/8" major diameter, so easy to find raw stock. McMaster and other industrial suppliers will have 5-40 taps and dies in all kinds of flavors, including forming taps. OCG makes really nice dies, which I get from Zoro.com.

 

[wazrus]

Standard threads for ME? Not usually, but there is a Model Engineer 'standard', called 'ME', which has thread pitches of 60 TPI in some sizes. 40 and 32 TPI are quite common in that system. All assuming it's actually available. Personally, I have a set of ME tackle, taps and dies and really, the system is only of real use with very thin walled tubing, or where there just has to be a fine pitch. I did, in my earlier years of ME., use British Standard Brass, which is a constant pitch of 26TPI, all sizes, Whitworth form. But BSB is very scarce, too and on the 'net, I've been able to source and I do have Metric in a range of sizes from 6mm to 38mm, 1mm pitch, metric form, both taps and dies. Quite reasonably priced and the tools themselves are quite well finished and certainly do the job. The 'net supplier was also able to provide a set of 0.5mm pitch tackle, from 6mm to 20mm: I could've had more, larger, but the 1mm pitch stuff suits most applications, so the very great majority of my efforts use metric, from 1mm., up to the larger one I mentioned, at 1mm pitch. Of course, the pitch in the smaller sizes (under, say, 6mm) varies, but that's not usually an issue. I have also sourced socket head capscrews, in 1.6 and 2mm and I do have some 2mm hex head setscrews. Nearly all my weeny little screws are Asian sourced. 3mm hex heads should be easily come by.
Standard? there isn't one.

 

[Jasonb]

I suppose a lot comes down to where in the world you are. In the UK BA is still very common on all the traditional designs and just like 5-40 on 1/8" stock the usual for that would be 5BA, 7BA for 3/32" stock and 2BA for 3/16" stock. It's what I tend to use on an engine that has imperial drawings and materials but if designing my own I tend to do it all in metric with metric stock and metric fasteners.

Fine threads for things like steam fittings, oilers etc are ME 32 and 40tpi on imperial models and metric fine or constant pitch on the metric ones.

Larger pipe tends to be BSP which is very similar to NPT

Whatever I'm using I do take the hex head size into account and seldom use standard sizes of metric usually opting for one size smaller as like UNC the standard hex looks too big on an period engine.

 

[Richard Hed]

There are several reasons I would not use British BA. ONe, because I would have to stock up on all the taps and dies. two, because the thread shapes are not 60deg but 55 which really isn't a better shape at all, in MNSHO. If I wanted to cut single point threads, I would have to shape the point to gawd awful rounded point at 55deg. There are plenty of sizes of bolts in UNC and UNF and even UNEF which can be found in most places. Usually UNC is good for most projects, but occasionally, one wants UNF.

 

[Charles Lamont]

Richard, Whitworth form is 55 degrees. BA is 47-1/2 degrees. Opinions will vary depending on location. Here in the UK we are probably as well off as anywhere for mulitiplicity of choice of threading tackle, with the first standard in Whitworth and its derivatives and having mostly moved to metric. There was also an awkward post-war period in which Unified threads were popular. I have a selection of taps and dies collected over many years variously in Whit, BSF, UNC, Metric, BA (0-10 + 12), BSP, NPT, Whit form constant pitch series in 60, 40, 32 and 26 TPI and a few oddments.

 

[Richard Hed]

Richard, Whitworth form is 55 degrees. BA is 47-1/2 degrees. Opinions will vary depending on location. Here in the UK we are probably as well off as anywhere for mulitiplicity of choice of threading tackle, with the first standard in Whitworth and and derivatives and having mostly moved to metric. There was also an awkward post-war period in which Unified threads were popular. I have a selection of taps and dies collected over many years variously in Whit, BSF, UNC, Metric, BA (0-10 + 12), BSP, NPT, Whit form constant pitch series in 60, 40, 32 and 26 TPI and a few oddments.

Thank you for that. It makes it even worse

 

[Peter Twissell]

For my own designs, I am perfectly happy to mix whatever threads are appropriate and available, including BA.
The majority of threads on models are oversize and therefore understressed
The engine I am building now has mostly metric threads, but a couple of BA threads, just because I have the tools available.
Metric taps under M3 appear to be available in two categories, very expensive or very poor quality.

 

[JLaning427]

Do yourself a favor, before you start building.

Throw away those cheap taps and dies! And maybe drills too.

Buy new (sharp) high quality stuff, in the sizes you are going to need.

Those (little) taps will be delicate enough already, you don't need to make your life harder. Getting broken taps out (without Ram EDM) is a royal pain, and often results in scrap.

As mentioned before, McMaster and MSC are both good sources for quality stuff. There are others sources too.

Lastly, consider what kind of taps you want. Spiral point taps are great for through holes. I even use them for blind holes (but then you have to clean the chips out) as I find they cut much easier than standard taps, and they're likely a bit stronger (usually 2 flute vs 4, so more material at work). There are other styles of tap as well. Some bring the chips up. Some form the threads by displacement. All have uses. Most are better performing than standard taps, but you will pay for that performance.

James

 

[methuselah1]

Razzle, the proposed model engineering standards were constant pitch 32 and 40 tpi. There are also 0.5, 0.75 and one millimetre constant pitch series for metric workers. These were put to the British Standards Institute, and can be found as P.D.6507/1982.

One enterprising manufacturer made a 60 tpi series available, but these didn't take off with modellers. As Charles mentioned above, there are other constant pitch series, both metric and imperial.

B.S.B is a constant pitch series (26 tpi) that became obsolete in the 1950s. It was a great favourite of E.T.Westbury, and as I'm a big fan of his engines, I've got quite a stock of the taps and dies, which are still readily available in the UK.

British Association was (surprisingly) never a british standard at all. It is derived from the swiss Thury thread, and was very widely used here. The funny flank angle reflects that the screws, for their size, are pulled up very tightly in this series. The sizes are nicely spaced, each one having 90% of the diameter and pitch of the previous one; this is unique among thread systems. B.A. is metric, but denoted in inches. Not a lot of people seem to know that. Like the bureaucrats that got rid of it, for example.

The threading tackle is easily obtainable down to 12 B.A. (52 thou) but any smaller than that, you would use the Progress system- the same as horologists do.

Of, course, if suitable and safe, there is no reason not to use whatever threads you like! I use 3mm HT capheads for holding on my con-rod caps, although nowhere else. No-one has mentioned UNEF yet, either.

So points for consideration are these.

M.E, B.A. and B.S.B. are readily available in both HSS and carbon steel. Carbon steel is actually harder than HSS, but it cannot take heat. Hand taps and dies do not get hot, and carbon steel is also cheaper. Don't buy taper taps; you'll never use them.

Fasteners are readily available, and are usually manufactured by the model engineering supply houses from hex stock, and are superior to stamped commercial items. Prices are acceptable for all but the very smallest sizes.

Many popular designs used M.E. and B.A. so you don't have to convert everything before starting a piece of work, so there's one less opportunity to make mistakes.

Taps and dies can have an exceedingly long life in a modeller's workshop, so even if importing them, the cost, over time, is small; especially so if you buy these things as you need them. I have been modelling in earnest for three decades, and it's only been a year or two since I finally "completed" my BA collection and started buying BSB.

I hope that helps.

-Andrew UK

 

[methuselah1]

I should have mentioned... If you break a carbon steel tap, you can anneal it, to help you get it out. If you break one in brass, you can often loosen it by applying a strong solution of alum and leaving the work in your airing cupboard for a week.

-Andrew UK

 

[Bentwings]

I am sure that this question gets asked repeatedly, but I am asking again. I am about to embark on my first small steam engine build from a set of castings i got on ebay. The 'Little Oscillator' found here: Steam Engine Price List | Precision Service I know that plans usually list out the fasteners needed for that engine, but I need to start purchasing taps and dies to make fasteners or start stocking up on fasteners to have on hand. Being in the US it can get tricky. Do I use UNC, Metric, or BA? The choices have large ranges of pricing for quality. Is there a list somewhere that shows the standard sizes for model engineering in each system? Is there any benefit of one over the other? Am i making this too complicated? Thanks for the input.

So far all of my contacts have been in Eupoe other than right here many us very small stuff 1/16 not 1/8 not very small metric threaded fasteners too.

mcmaster Carr has not everything needed in the USA

I have bough lots of fasteners from micro fasteners they supply airplane and RR modelers. Hobby shops usually have down to 2-56 shcs and others the Rc car parts have many metric fasteners very small taps and dies can be tough to use if you get hole size too tight or round stock a little oversized. I’ve done 2-56 threads on the lathe but not with this messed up eyesight it take sharp well honed tooled and near perfect feed rates especially in gummy or stainless steels brass and bronze need good chip cleaning stainless work hardens quickly so tools need to very good tough and sharp don’t forget cutting oils

 

[delalio]

Hi all,
Just my very limited 2c on the subject.

I'm also very new to model engineering. I had nothing 3 years ago, so all my purchases are recent, and on a budget.
You say you have a miniature steam engine to build. Do you want the bolts to look to scale? Do you have any other projects in mind after this one?

I think for anyone getting started, it doesn't matter what you decide, you'll probably spend your first few years constantly needing just one more tap/die to complete a project.

That said, unless you are doing very specific work, most threads will be physically strong enough for most applications, so the major diameters and the stud/bolt size will be the thing that makes a model look "right". How many turns you have to do to get there (TPI) is mostly irrelevant and mostly hidden once the model is built.

I'm in the UK/Ireland. Here we are well serviced, like others have said. I assume you are equally well serviced in the USA? Your profile says TX? The internet also allows you to get pretty much anything if you know the right places to look!

Personally, I went with BA and ME threads. I bought full sets of their HSS taps and dies from Tracy Tools, here in the UK. No affiliation, YMMV, but I've found them to be quick, reliable and friendly! They also have stands at a lot of the model shows here and the prices are very reasonable too, and they're pretty local so have good delivery times.

I picked BA because most of the models here use that thread size for the structural parts to the models, (Bolting to the base, bolting parts together etc) and it looks to scale.
I also use ME threads because a lot of the kits and suppliers here use these for pipe fittings and valves, off the shelf parts, etc - Anything connecting boiler bushings, valves, pipe joints and tees, etc seems to mostly be ME threads.

Those two sets worked for about 85-90% of all the work I've wanted to do, and covered all the mini steam models I've built. Total cost ~ £180.

Other things I've needed are a few 26tpi taps of differing sizes, as I recently acquired a 7 1/4 steam loco, and some of the safety valves use the slightly thicker thread.

Now, I've also done some work on newer things. I made a few fountain pens etc, and many of them use obscure metric threads, like M9 0.75 pitch.

Like others have said, get good quality tools when you buy. Be careful with them, and always use cutting fluid where recommended. I went in dry a few times and regretted it very quickly!

So, in short, look at the types of off the shelf items you will be using for your model builds, and when you buy, buy a full set.
I hope that this is at least some help.


Kindest Regards,

Del


I did a very brief bit of research, and from what I can see, thread sizes in the USA seem a bit messed up for model engineering. Here in the UK, there seems to be a lot less variance!
For example, this uses ME threads : Displacement Lubricator - PM Research (pmmodelengines.com)
but then these uses a whole range of threads: Globe Valves - PM Research (pmmodelengines.com)
Boiler Feed Pump - PM Research (pmmodelengines.com)

 

[razzle]

This has turned into quite the educational discussion. Lots of good advice and supporting information. Thank you. It looks like I will be investing in some taps and dies over the next few years. I will probably start with imperial first. The pick up the others as needed later.
Should I buy spiral flute or get 3 piece tap sets (straight, plug, bottoming)?

 

[Jasonb]

I've gone over to spiral flute for most of my commonly used sizes and will replace any others with the same should the need arise. Although designed really for blind holes they work just as well in through holes with only the very small size needing to be backed out occasionally as there is not much room for the swarf to spiral up out the hole as it does on the 3mm and larger ones.

They are also good if you want to power tap in the mill or lathe

 

[methuselah1]

Hi all,
Just my very limited 2c on the subject.

I'm also very new to model engineering. I had nothing 3 years ago, so all my purchases are recent, and on a budget.
You say you have a miniature steam engine to build. Do you want the bolts to look to scale? Do you have any other projects in mind after this one?

I think for anyone getting started, it doesn't matter what you decide, you'll probably spend your first few years constantly needing just one more tap/die to complete a project.

That said, unless you are doing very specific work, most threads will be physically strong enough for most applications, so the major diameters and the stud/bolt size will be the thing that makes a model look "right". How many turns you have to do to get there (TPI) is mostly irrelevant and mostly hidden once the model is built.

I'm in the UK/Ireland. Here we are well serviced, like others have said. I assume you are equally well serviced in the USA? Your profile says TX? The internet also allows you to get pretty much anything if you know the right places to look!

Personally, I went with BA and ME threads. I bought full sets of their HSS taps and dies from Tracy Tools, here in the UK. No affiliation, YMMV, but I've found them to be quick, reliable and friendly! They also have stands at a lot of the model shows here and the prices are very reasonable too, and they're pretty local so have good delivery times.

I picked BA because most of the models here use that thread size for the structural parts to the models, (Bolting to the base, bolting parts together etc) and it looks to scale.
I also use ME threads because a lot of the kits and suppliers here use these for pipe fittings and valves, off the shelf parts, etc - Anything connecting boiler bushings, valves, pipe joints and tees, etc seems to mostly be ME threads.

Those two sets worked for about 85-90% of all the work I've wanted to do, and covered all the mini steam models I've built. Total cost ~ £180.

Other things I've needed are a few 26tpi taps of differing sizes, as I recently acquired a 7 1/4 steam loco, and some of the safety valves use the slightly thicker thread.

Now, I've also done some work on newer things. I made a few fountain pens etc, and many of them use obscure metric threads, like M9 0.75 pitch.

Like others have said, get good quality tools when you buy. Be careful with them, and always use cutting fluid where recommended. I went in dry a few times and regretted it very quickly!

So, in short, look at the types of off the shelf items you will be using for your model builds, and when you buy, buy a full set.
I hope that this is at least some help.


Kindest Regards,

Del


I did a very brief bit of research, and from what I can see, thread sizes in the USA seem a bit messed up for model engineering. Here in the UK, there seems to be a lot less variance!
For example, this uses ME threads : Displacement Lubricator - PM Research (pmmodelengines.com)
but then these uses a whole range of threads: Globe Valves - PM Research (pmmodelengines.com)
Boiler Feed Pump - PM Research (pmmodelengines.com)

Wow! PMR could confuse anyone- that's before you read of their "let's make it up as we go along!" Series...

-Andrew UK