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Locomotive threading

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Gary Clarke 405/05/2022 17:20:51
22 forum posts

Hi All,

I am in Canada. I have a question regarding threading for 3.5 and 5" british locos. The threads on my builds all call out for ME or BA threads mostly. fasteners in these threads (and taps/dies too) have to come from UK. Any concerns or suggestions for alternatives that may be more readily supported in my neck of the woods?


John Purdy05/05/2022 17:38:06
360 forum posts
204 photos


Here's the conversion chart I have developed to convert BA to UNC/UNF threads which I have used on my projects. I normally make all my hex headed screws and nuts from hex stock. If you want I can email you a PDF of the chart if you PM me your e-mail address.


dscn2959 (2).jpg

Edited By John Purdy on 05/05/2022 17:41:39

Edited By John Purdy on 05/05/2022 17:54:06

Clive Foster05/05/2022 17:43:14
3172 forum posts
113 photos

For all practical purposes metric or American (unified to Brits) of close to the same diameter are a perfectly adequate substitute for BA threads. If easily available metric fine is a better alternative than ordinary metric. The shallower thread of BA is an advantage in small work. If nothing else it makes the life of a small tap easier!

Similarly the prime advantage of ME is the shallow thread depth. Which helps when getting the proportions of small fittings et al right. Metric fine or AF / UNF will usually substitute fine but you need to watch the core diameter if its a hollow component. I've had ordinary metric pretty much turn into a spring when I wasn't paying attention.


PS John posted his excellent chart whilst I was typing.

Edited By Clive Foster on 05/05/2022 17:44:34

JasonB05/05/2022 18:30:08
23039 forum posts
2769 photos
1 articles

having built quite a few American sourced engines my chart is the other way round but easy enough just to look at the BA colours and choose a suitable UNC or UNF number.

Revised thread conversion

You may also need to look at UNEF and the Model Pipe series for some of the finer threads

Also be aware that standard UN fixings will have quite a large hex so may not fit the space available but you can get small hex from the likes of American Model Engineering, see this and the next page. When you click on the screws it shows a chart of the hex sizes

HOWARDT05/05/2022 20:14:10
932 forum posts
39 photos

In the UK it is cheaper to use metric threads for fasteners bought in the hundreds not in packs of ten. As the pipe fittings are based on small sizes then it is quite easy to work with ME threads and make fittings as industrial fittings in both metric and asp threads tend to be quite large. Living in Canada as you are then stick with what you can purchase locally, when I was designing machines for use in Canada some years ago metric fasteners where no problem. With your closeness to the USA you may get better prices by using UN threads. AT the end of the day it doesn't matter what thread form is used but as has been said you have to look at the AF of the fittings for fitting clearance.

CHAS LIPSCOMBE05/05/2022 23:06:48
34 forum posts
3 photos

From Jason's chart, I see that 1BA is something of an "orphan" thread with no clear alternative. A nuisance because it was used extensively by Amal carburetters for their motorcycle handlebar levers and ealy carbs. Does anyone know of a closer equivalent. than the ones shown in Jason's list?

Dennis R05/05/2022 23:35:51
73 forum posts
16 photos

UNF 12 x 28 which is only 0.007" bigger and 0.2 pitch difference

JasonB06/05/2022 06:57:52
23039 forum posts
2769 photos
1 articles

As 1BA is not commonly used for fixings being a non preferred size I did not colour match it to anything. But if you look at the numbers and just choose the nearest in the system you are using which will be M5 or #12 as Dennis says.

The #12 being closest with a 0.007" difference, M5 a little more at 0.012" smaller ( 0.3mm)

1BA is also common on gas jets. Which I would rather get 1BA taps and dies for than try and drill such tiny holes. For replacement fixings on Amal parts then you would also want to cut matching threads, or if the casting has stripped you will be wanting a bigger replacement screw anyway. These charts are more for making from scratch than restoration work.

Edited By JasonB on 06/05/2022 07:23:59

Mike Poole06/05/2022 07:11:00
3376 forum posts
77 photos
Posted by CHAS LIPSCOMBE on 05/05/2022 23:06:48:

From Jason's chart, I see that 1BA is something of an "orphan" thread with no clear alternative. A nuisance because it was used extensively by Amal carburetters for their motorcycle handlebar levers and ealy carbs. Does anyone know of a closer equivalent. than the ones shown in Jason's list?

1BA taps and dies are widely available in the UK but probably harder to find and expensive in the rest of the world. Unless you are making both parts then it’s probably best to stick with 1BA if possible. Using non standard sizes will be setting a trap for anyone who has to work on the machine in the future.


Howard Lewis06/05/2022 18:59:44
6305 forum posts
15 photos

The purpose of a thread is either to provide a load to clamp components together, or a fine adjustment.

So, as close an approximation as you can get in another thread system will do the job just as well.

The easiest is M6 instead of 0BA, since then only difference is thread form.

UNF and UNF are easy equivalents for BSF and BSW.

NPT tpi usually differ by one from BSP, as well as the thread form, but are intended to perform the same function.

ANC and ANF threads should provide suitable alternatives for some of the smaller BA sizes, as would some fine pitch Metric threads.

For the latter you might have to import Taps and Dies from UK traders like Tracy Tools, or The Tap and Die Company



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