Are there 'Standard' thread sizes for model engines?

[ShopShoe]

Philosophically,

I have built up my tap and die collection over the years the same way I have bought drills: By project, more or less.

I am in the US, so UNC and UNF are what I have the most of. Since I also use metric stuff, I also have been building up my collection of metric taps and dies based on what my projects call for. I also have occasional "Odd-Size" items such as some taps for British piping as used on old British cars and the 5/8-27 that audio guys know is what microphone stands use here.

I have not got any BA, ME, or MTP as I haven't made any scale models: most of my engine stuff is like Elmer's Engines and made-up stuff, which is for me and not for show. If I start down the path of casting kits, I suspect those may show up.

I generally buy good quality at prices I can stand, but I have some cheaper items just so I have something I might need: For example, I have good metric taps and dies bought for the sizes I have needed, but I also have a discount-house metric set to fill in the gaps in my range. I tend to buy taps in multiples when projects are planned, sometimes buying the same size in more than one style or brand or quality, so I am slowly learning what I really want to buy the next time. I have spiral taps in sizes I use a lot, as I can power-tap with them or they work well in harder materials. I also have some forming taps in small sizes I haven't really used a lot yet, as I have been sidelined from building more involved things due to famiily events.

One thing I like to emphasize is that I have high-quality drills for use with the taps only, and I buy them in multiples when project planning.

I believe taps should be considered supplies and good ones kept carefully and ones that are getting tired should be retired (not thrown out, just not used unless there is no other option.)

--ShopShoe

 

[Jasonb]

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)

The ones in your first link are actually UNS (unified special) which is a 60deg thread form not 55Deg as is ME

The ones in the second link also have the option for taper threads and pipe threads. One needs to bear in mind that in the US most models are plumbed up with the pipe ends threaded and screwed into fittings, elbows, tees etc. so these are more suitable for that.

In the UK we tend to bend our pipe around corners and then use nuts and nipples (ferrules) to join them to steam fittings, these fittings tend to use ME threads at least in the smaller sizes and BSP or 26tpi as they get larger.

Over the channel all their boiler fittings tend to be Metric fine or Metric constant pitch series and the nuts and nipples for metric tube or G thread as they get larger.

So I would not say PMR are mixed up, they simply supply what their domestic market tends to use and helpfully tweak the threads so that those of us in different parts of the world who also use them can get away with using our existing threading tackle. Bit about their threads here Model Pipe Threads - PM Research

 

[methuselah1]

The ones in your first link are actually UNS (unified special) which is a 60deg thread form not 55Deg as is ME

The ones in the second link also have the option for taper threads and pipe threads. One needs to bear in mind that in the US most models are plumbed up with the pipe ends threaded and screwed into fittings, elbows, tees etc. so these are more suitable for that.

In the UK we tend to bend our pipe around corners and then use nuts and nipples (ferrules) to join them to steam fittings, these fittings tend to use ME threads at least in the smaller sizes and BSP or 26tpi as they get larger.

Over the channel all their boiler fittings tend to be Metric fine or Metric constant pitch series and the nuts and nipples for metric tube or G thread as they get larger.

So I would not say PMR are mixed up, they simply supply what their domestic market tends to use and helpfully tweak the threads so that those of us in different parts of the world who also use them can get away with using our existing threading tackle. Bit about their threads here Model Pipe Threads - PM Research

Razzle, I have just checked the UK "big hitters" in our modelling scene... These are-

Reeves 2000
Stuart Models
GLR Kennions

And also,

Macc Models
Steamfittings.co.uk

(Usual disclaimers, of course, and there are many others.)

Macc Models produce one safety valve threaded 1/2" x 26, otherwise none of the above sell any drain cock, boiler bush, union cock, displacement lubricator, or oil cup- in fact nuthin' that isn't threaded 1/8, 3/16, 1/4 or 5/16" ME. Nothing metric to be found, either.

As I have mentioned, many of our best designs are imperial, as are many of the recent ones (c.f. Anthony Mount) and continue to use the old, proven threads, to save modellers from having to re-stock. Dual dimensioning is, however, becoming more common. Thus far, I have yet to see a metric thread specified that has not been from the everyday ISO Coarse Series, though.

The ghost in the machine; if you measure the shank of an M6 tap, you'll find it is 1/4". Why? To avoid retooling costs, forty-two years ago.

I have recently left a university post, teaching in the machine shop, and the under (and post) graduates have so little time allocated to practical experience, that it is hard to imagine any of them joining the hobby. The old ways will persist!

-Andrew UK

 

[Jasonb]

Yes if you stick with the usual suspects that tend to cater for stationary engines and upto 7 1/4" locos they will mostly be ME but for those that make traction engines or 1/2 scale gas engines something a bit bigger is needed and catered for by the likes of R A Baker or LSM in the UK.

Plenty of IC engine models with fine and constant pitch threads for their spark plugs such as the commonly use CM-6 with it's M10 x 1 thread, even Westbury's drawings have those

Anthony Mount went to dual dimensions and his last few are all metric, I also have M6 taps with 6mm nominal shank, suit modern ER tapping collets with their square register but do also have ones with 1/4" nominal shank which may be better suited to those using old tapping heads. Just buy what you need or want to use.

 

[Bentwings]

I was going to ask the same question I just ran into. 40tpI ME I assume it’s metric pipe of some sort. My machinery book is out at the shop so other than look on internet I’m in the dark many of the small steamers have metric threads but the pipe threads are new to me. I found an oiler with unusual threads just a few minutes ago. I’d hate to spend this much on a nice engine and have to cobble a fitting
Byron

 

[Bentwings]

I was going to add this to my reply but I think it’s better separated . I looked up steam oil and lubes Yess there are specials they have small amounts of tallow in them had to do with keeping lube on the surface even with water or steam around . So I looked where to buy. Well you can get 50 ml bottle when you buy the engine as an accessory . I did find commercial stuff but s 5 gallon pail will far out last the engine and me I didn’t ask the price. I’d think gear lube would work but oh man the stink in the house . So far I think marvel mystery oil might be good alternative for light oil . Not sure about seafosm wd40 probably not as it gets under water pretty well . Motor oil might be ok just messy . I already have a plan for model mess. One thing I have noticed is that these model engines don’t have the best oil or lube systems. You probably have to keep after them I remember using my moms sewing machine oil which she didn’t like . I have an industrial sewing machine snd I use mineral oil on it. It doesent stain fabrics I guess . Leather soaks it up and it disappears . I’ve heard that model steamers use it but I haven’t explored it much yet .
so oil is the question what’s good ?
byron

 

[Jasonb]

I was going to ask the same question I just ran into. 40tpI ME I assume it’s metric pipe of some sort.

No that would be for a pipe or other item with an imperial OD eg 1/4", 3/8", etc and it has a tpi (threads per inch ) of 40. The ME thread series are all imperial based on OD. These are quite often use don pipe as the high tpi means the thread is quite shallow so you can get away with a thinner wall tube.

As for oil, if you are going to steam it then yes steam oil is best but if running on air it can be a bit thick and sticky at room temperature so better to use a lighter oil for air running. Again depending on the size of engine 32wt oil may do for a reasonable size one but the very small I tend to use a clock oil which would be much like your sewing machine oil. We can buy steam oil here in the UK in many sizes from most of the specialist model engineering suppliers usually in two weights.

 

[Bentwings]

I was digging through some hobby stuff and I ran across an old Badger airbrush. Lightbulbs went off. What a neat way to spray small amounts of oil especially steins slide valves snd hard to get to places I have a bunch of nozzles hose snd fittings. Super flexible air hose and a dozen little bottles to put various lubes in I suppose a bottle rack would be in order but not a problem . I even have an old compressor . The thing still works. It must be 20 years old or more you could thin the heavier oils out with acetone but don’t spray around open boiler flames . I’m going to put it all back in a new box and label it can’t wait to try it out. I’ll take it with me next time I go to the hot rod shop . It would be perfect on the lathe or mill to keep the oil mess down
Byron

 

[Jasonb]

The mist created by spraying coolant/lubricant is far from perfect.

 

[methuselah1]

I was going to add this to my reply but I think it’s better separated . I looked up steam oil and lubes Yess there are specials they have small amounts of tallow in them had to do with keeping lube on the surface even with water or steam around . So I looked where to buy. Well you can get 50 ml bottle when you buy the engine as an accessory . I did find commercial stuff but s 5 gallon pail will far out last the engine and me I didn’t ask the price. I’d think gear lube would work but oh man the stink in the house . So far I think marvel mystery oil might be good alternative for light oil . Not sure about seafosm wd40 probably not as it gets under water pretty well . Motor oil might be ok just messy . I already have a plan for model mess. One thing I have noticed is that these model engines don’t have the best oil or lube systems. You probably have to keep after them I remember using my moms sewing machine oil which she didn’t like . I have an industrial sewing machine snd I use mineral oil on it. It doesent stain fabrics I guess . Leather soaks it up and it disappears . I’ve heard that model steamers use it but I haven’t explored it much yet .
so oil is the question what’s good ?
byron

For bearings and cross heads, I've always used whatever my car or lawnmower was having, without ill effect. I have a 60 y/o car, and I don't use synthetics. There has recently been a thread here explaining what oils will hurt bronze- worth looking out. I think the guy was calling out certain grades of gearbox oil. Steam engineers would often use bronze piston rings, and betray their backgrounds when beginning in i.c!

Don't use 3 in 1- the lighter fractions evaporate over time, leaving a gummy deposit; save it for your garden shears.

Steam oil is a different animal, due to temperature and pressure. People like Stuart models sell it by the half litre (about 7/8 of a UK pint) and even if not cheap, it'll last a long, long time... Plus which you can also decant it and sell it to like-minded friends. Theirs is designed for displacement lubrication, but works just as well with mechanical lubricators- and gives that wonderful smell that your wife will probably still complain about, but you'll find music to your nostrils....

 

[methuselah1]

Yes if you stick with the usual suspects that tend to cater for stationary engines and upto 7 1/4" locos they will mostly be ME but for those that make traction engines or 1/2 scale gas engines something a bit bigger is needed and catered for by the likes of R A Baker or LSM in the UK.

Plenty of IC engine models with fine and constant pitch threads for their spark plugs such as the commonly use CM-6 with it's M10 x 1 thread, even Westbury's drawings have those

Anthony Mount went to dual dimensions and his last few are all metric, I also have M6 taps with 6mm nominal shank, suit modern ER tapping collets with their square register but do also have ones with 1/4" nominal shank which may be better suited to those using old tapping heads. Just buy what you need or want to use.

Sparking plugs have always been difficult. Westbury's 1930s design, which was later to become the Kiwi II, originally specified an M12 special. The plugs were probably cheap enough, but the taps? After Pacy and Champion stopped making 3/8" plugs, we were stuck with M10 x 1mm (that's actually fortunate- they're cheap); even if you have the sponds to buy Rimfire plugs, they use a UNS thread, so you still need to buy a special tap!

And forget getting old Pacy or Champion plugs now- they have their own collectors' market...

 

[wazrus]

The 'quality' message keeps cropping up in more than few areas of engineering, not just 'model' engineering. What's the 'quality' really worth and isn't a lot of it to do with something akin to 'national pride' or maintaining the status quo? I have mentioned my threading tackle as including metric, and metric can be extremely expensive. Nor is metric free of confusion. Take 12mm., which is generally available in five 'standard' pitches: the situation is not unlike that with the plethora of so-called 'imperial' systems. Five commonly used 'thread types' for the one size, so there's no loss or gain, is there? Like it or not - and I wasn't particularly keen about buying lots of new tackle - metric is here to stay. So for me, the issue is one of dollars and/or pounds. Quality? Just how is that assessed? By the manufacturer's say so? By 'standards? Or perhaps the price is a guide to quality? Tell me another one!
I regularly and routinely use taps and dies down to 1.4mm and while there are breakages, the Asians vs 'reputables' show no differences in durablity or finish, but if a 38mm tap is priced at $A800-odd and the Asian at $A100, then one is well advised to ask about the differences. Sure we can bat on about supporting the local industries, and that's laudable, but what does a non-sale at $800 support? I simply won't buy it! And if I had to, I'd certainly be factoring those sorts of prices in: it's a bit of a circular argument, isn't it?
I don't rush to Hong Kong to buy my stuff, but more than a few visits there have shown Australian (and perhaps others' countries) retail margins as being pretty well obscene. Four to five hundred percent is not unusual. And we, as model engineers with our very small quantities, are very much one of the retailers' targets. Then there's the 'scale' appearances' considerations and for this I do keep some very old BA hex heads, but only as decoration. They are in brass and a bit of torquing might shear 'em off: I'm sure I'm not alone in this! Enter the Asians, where I can have fasteners in brass, mild steel (zinc plated) or stainless steel and there was one occasion, in HK., where I had asked about stainless 2mm hex head screws (not bolts) and was surprised when asked how many thousand would you like, sir? Now the heads aren't as small as 'scale' might dictate, but if the whole project is 'metrified', what's the loss? And the SS can be better torqued up for steam tightness.
Last, but not least, is the 'net and there are firms who'll do almost any nut or bolt you might think of and there are others who'll supply taps and dies by the bushel and those bushels will be very comparable, in price and quality, to the single purchase at the 'shop'. You might counter with that perhaps you don't need 1,000 of xyz screws, but if the price is the same as ten offered at the 'shop', does the shop appeal for anything other than simple convenience? Then, they might have to 'get it in' for you. Oh, yeah.....

 

[Charles Lamont]

...
even if you have the sponds to buy Rimfire plugs, they use a UNS thread, so you still need to buy a special tap!
...

They are not that fussy. They fit a 1/4" x 32TPI ME (Whitworth form) tapped hole without the slightest difficulty.

 

[Jasonb]

And even if you don't want to use ME the UN taps are not exactly going to break the back, £2 from Tracy for a 3/8x24 UNF if using the Rimfire V and £5-6 for 1/4x32 UNEF or M12 x 1.25 if using Westbury's old plug size and if you are really tight that could be screwcut on the lathe

 

[methuselah1]

Whilst at the University, each year we built a racing car- the series was called Formula Student. We would on occasion have guests from other countries. One year, we played host to the Indian contingent, and they needed a track rod made up. They gave me their own taps.

Those taps were going to break before making a thread. They were brand new!

That's a quality problem.

 

[Geartooth]

You could do a quick spreadsheet program that divides diameter by your chosen scale. Multiply common thread pitches by the same scale. Use the answers to compare available standard thread sizes to find a usable scale thread. For example; in 1:3 scale, a 5-40 works for a 5/16-18 .

 

[Bentwings]

They are not that fussy. They fit a 1/4" x 32TPI ME (Whitworth form) tapped hole without the slightest difficulty.

I’m now finding some extra fine threads I think our lathe is good to 56 threads per inch. I can’t ever see that small any more withou mag glass. Try grinding tool bits with mag glasses and mag glass I can actually get a pretty nice honed finished but can barely read micrometer now I can use a tiny pic to count the divisions getting pretty good at counting to one less than 10 LOL stand back if I’m moving around. It’s unknown which way I’ll go. I don’t know what it will be like if I have to use a walker in the shop. I just found a site that teaches LIDAR collision avoidance so I’m learning about that now . I’ll have to equip my guide cat with LIDAR so we can get around Im
Not sure how sight impaired find dog poop I never got a good answer on that one electronic nose shaved developed to that level. Maybe I can use the LIDAR to follow the dogs nose because they always turn around to sniff the pile, but cats cover it up and do a pretty good job of it . Aircraft use this to follow terrain when landing so maybe I could learn to keep track of the fog or cars head , then look for the pile .
buron

 

[abby]

I have a large collection of BA stuff from 16 BA which I have never used up to 2 BA which I use very often. Here in the UK BA stuff is very readily available from the workshops of retired or deceased ME's so it makes economic sense to use it , I even have a Coventry diehead with several sets of BA chasers including 10 BA , and left hand taps and dies used when making adjustable length valve rods etc.
Different hexagon head sizes are also available for BA which can give a much better scale appearance.
For countersunk socket drive screws I use metric , M3 ,M2.5 and M2 with spiral HSS taps available quite cheaply from a UK company mentioned in the carbide insert discussion.
With the die head can I can cut a good 10 BA thread on a 1/16" mild steel rod if studding is required.
Dan.

 

[Bentwings]

Interesting I was toolmaker going to. Engineering school and changed jobs. The very first thing I had to make was a bunch of small stainless steel pins with 2/56 thread right in the middle the shop had 3 brand new Harding’s tool room lates whit every attachment made the shop mgr gave me a drawing with the part snd a hang full of tool bits and said see what you can do with this one . I had to make a special follower test to keep the things straight . There were a lot of them too as I remember they had a mating part that had 2/56 threads down in a hole so it was a tough project but good experience . It was a fun shop to work in as anything you needed was there as soon as possible . Or little Grizzly isn’t as good as the Harding’s but it does have power with the three phase motor. And it has good speed if you need it. Big issue is you can’t reverse the lead screw there is a conversion on the internet but it doesn’t fit our lathe . I have a reverser modeled up but not built yet. I’m more interested in this steam thing for now . My son has a couple giant scale Rc warbirds he is working on so we chat quite a bit

byron

 

[terryd]

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.

Hi Razzle,
It's a moot point but the'Imperial ' system is not that used in the US. Over there it is, I believe classified as 'American Standard'. There are some differences between the two, so best not to get confused. There are measures, especially in volumetric measurement that are radically diferent and you use a few measures that are archaic as far as the Imperial system is concerned.

TerryD