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Conversion table

American sized plans

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Tom Gullan15/07/2017 22:37:29
87 forum posts
34 photos


I'm looking for some help with converting American sizes to U.K. sizes.

After building several stationary steam engines I have taken the plunge and decided that my next build will be the Upshur Vertical "Hit and Miss" petrol engine.

Having purchased the plans, and studied them in detail, I am somewhat confused as to the screw fixing sizes and numbered drill holes that are used e.g. 6-32 thread.

Does anyone know if I can easily convert these American sizes to either BA or metric?




Edited By Tom Gullan on 15/07/2017 22:40:33

Edited By Tom Gullan on 15/07/2017 22:41:09

Edited By Tom Gullan on 15/07/2017 22:41:47

Nick_G15/07/2017 22:40:59
1808 forum posts
744 photos


Yes. .......... ish.

Jason sent me a link to a chart when I was making the Hoglet engine. Cannot find it ATM but he or others will no doubt be able to provide it.


Brian Sweeting15/07/2017 22:50:13
437 forum posts
1 photos

Tap Drill sizes..


Drill / metric sizes


Also a calculator if it helps.

Edited By Brian Ess on 15/07/2017 22:55:26

Frances IoM15/07/2017 23:15:11
804 forum posts
26 photos
refs like 6-32 refer to UNC or UNF (depends on tpi) size #6 with 32 tpi -sets of numbered drill bits are readily available as are UNC/UNF taps + dies tho metric fittings are now much easier to obtain on this side of Atlantic

Edited By Frances IoM on 15/07/2017 23:16:58

duncan webster15/07/2017 23:35:47
2734 forum posts
40 photos

I take it you're looking to use BA or Metric instead of UNC/UNF

Here is a comparison chart for metric


Unless you've already got BA screwing tackle I'd go for metric, the nuts and bolts are a lot cheaper, if not as aesthetically pleasing, the heads look too thin to me. Having said that youi'll struggle to find smaller than M3 hex bolts, at least I do

Frances IoM15/07/2017 23:41:18
804 forum posts
26 photos
I'm away from my workshop at present but if my memory serves a good supply for 6-32 small machine screws is old desktop computers - used where a M3 would probably be used in metric based equipment

Judging from costs at my local fittings supplier BA threads are generally very restricted in options + more expensive than UNC/UNF which in turn are quite a bit more expensive than metric
Clive Foster16/07/2017 01:16:15
2319 forum posts
76 photos

When searching for near equivalent sizes the best and most nearly comprehensive list to be found on the internet is that originally due to Andy Pugh. It presents a goodly selection of threads in simple nominal outside diameter size order. All threads are given in both imperial and metric units regardless of the official system. The original has been extended and re-formatted for easier reference by other workers.

Original text format file here :- **LINK**

Prettier Excel format version here :- **LINK**

PDF version arranged to print out on A4 pages here :- **LINK**

Don't recall where I found the one I keep on the computer. In Excel format and styled like the print out from PDF version except for a having the thread type designation data at the top. Its a single unpaginated list with only one row of column headers. Using a horizontal split screen display the designations and column headers at the top can be fixed with the sizes showing in a second scrolling window. Much easier than a long list.

Certainly not my re-arrangement because I always put "harder" breaks every 5 and 10 lines in long tables to facilitate reading across.


JasonB16/07/2017 07:54:40
18632 forum posts
2048 photos
1 articles

As Nick says I made up my own chart as I tend to make quite a lot of American engines. Highlighted threads are the usual ones I substitute depending if I'm using BA or metric. If you want to stick with imperial stock as per teh drawing then BA is the easier option as for example they will put a #10 thread onto a 3/16" rod and you can put 3BA onto that size material too.

Link to PDF so you can alter it as you wish lighter highlighting makes it easier to read once printed

Tom Gullan16/07/2017 21:45:11
87 forum posts
34 photos


Thank you for all of that useful information.


John Purdy18/07/2017 22:02:17
204 forum posts
69 photos


Here is a table that you might find of interest. I made it to convert BA to UN (I live on the west side of the pond), but in can just as easily go the other way. It also gives the closest imperial size hex for the one size smaller ME heads and nuts. I'm not sure if you are aware but if you look at the last column (major dia. of the UN sizes) you will see that a #0 machine screw is .060" dia. and each successive size up is .013" larger. (Odd numbers above 6 are no longer produced).

Johndscn2959 (2).jpg

Tom Gullan20/07/2017 00:32:59
87 forum posts
34 photos

Thank you John.



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