In the UK my guess would be BSW, in North America my guess would be UNC, I use M6. As far as I know the UK mostly used Whitworth (55 deg.) threadform up to the 1970s, US made items that were shipped to the UK during WW 2 was 60 deg., more here.

Thor

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Whoever told you should have made it clear

If Whitworth thread-form is required, then it should be specified as 1/4” BSW

If [as seems likely] Unified thread-form is required, then there should be a ‘Tolerance Class’ appended

MichaelG.

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Edit: __ From reading your latest post , UNC does seem most likely, and in the absence of proper specification, probably class 2 … 

Edited By Michael Gilligan on 20/02/2022 08:37:29

For 1/4 x 20 UNC the tapping size is 5.20 mm

For 10 – 24 ANC the tapping drill is 3.90 mm.

Howard.

1/4" x 20 is UNC or maybe ANC, its American ancestor.

Although 1/4" Whitworth is also 20 tpi the designation is 1/4" BSW.

British practice has always been to name the common range of standard "tap'n die for fixing things together" threads so you get a diameter and a letter code that is a contraction of the name. In the UK if it a UK native specification thread has a x TPI suffix applied it is in greater or lesser degree specialist. Cycle is probably the only one that has an x TPI and can legitimately be considered common enough to be non-specialist.

Correct common UK practice would be 1/4" UNC for 1/4" x 20. This should also apply to the smaller number series, preferably with a No prefix. No 6 UNC for example. The potential for confusion with BA is obvious. American # prefix is probably clearer.

However most references tend to follow the Zeus book practice by tabulating the numbers as either ANC for coarse and ANF for fine. Which is what they are. The UN series imported the American specifications, as tided up for wartime use, wholesale for the smaller sizes.

American style number – TPI designation is common in the UK too and probably as correct as anything. As far as I know BSI is silent on this but there is probably a military standard that ought to be definitive.

American practice tends towards diameter x TPI or number – TPI, often with # in front, although applying ANC, UNC or one of the other less common specification identifiers isn't uncommon. Usually only done when it matters as the differences are of littile import for standard fastenings. Tables of sizes are generally headed NC, NF, ANC, ANF, etc as appropriate.

Tolerance classes are rare in normal bolt things together work whatever the thread. In practice equivalent to modern h6 / H6 class is assumed. Probably class 2 as Michael says.

Particularly when it comes to wartime production some of the tap and die designations can be – ahem – odd. If I've not broken it I have a BA tap lurking somewhere with the diameter in decimal inches and a pitch in mm! Also had / have some Whitworth and BSF sizes with the exact diameter in decimal inches. A few thou off nominal size. All American made from quality firms. I suspect that in the days before tolerance classes exact size designations were the American way to get tight or loose fits as needed. presumably for plated items.

Clive

Edited By Clive Foster on 20/02/2022 09:22:35

If the bolt and hole are both new, there's no particular reason to follow the instructions exactly. They're probably intended to make life easy in North America and being confusing outside the USA is an accidental side effect.

As Metric fasteners are available almost everywhere, it may be appropriate to substitute the nearest metric size because taps, dies, and fasteners are all easier to source in metric. 1/4" is close to M6 and #10 is close to M5.

The exception is when a bolt has to fit into an existing thread, or a new hole has to be threaded to fit an existing bolt. Happens a lot when repairing machines. In that case the original thread specifications should be followed.

But when the need is just to join two lumps of metal together, you can use whatever thread system is convenient locally. 1/4" BSW isn't ideal because it's an obsolete British thread. Could be substituted for UNC 1/4 – 20, but then you have to find a BSW bolt to fit the thread. Though the internet is helpful, it may not be easy to buy obsolete British fasteners.

Dave

I may be legitimately accused of ‘reading between the lines’ Dave … but I think the holes need to be tapped to suit already ‘bought-in’ items.

[quote] The thread sizes used are 1/4”-20 at the knobs … [/quote]

MichaelG.

You can use your 1/4 BSW tap and a 1/4 UNC bolt or other thread will screw into the resulting thread just fine. They are the same TPI, ie 20. The slight difference in thread form angle is negligible in most real world applications and the two are commonly screwed together without problems the world over.

For all practical purposes in the home workshop, BSW and UNC threads are interchangeable on those sizes that use the same diameter and TPI, which is most of the ones commonly used such as 1/4, 5/16, 3/8 etc, but annoying not all. Check online thread charts for the crossover sizes.

Edited By Hopper on 20/02/2022 10:37:19

Whilst on the subject of threads what is the difference between BSB and BSC both are 26tpi ? does one have a different thread angle ? Noel.

Yes it might be wise to use 6mm if you can make both halves of the job and fit them together.

I go through very similar ructions here in Australia. BSF in particular is getting very hard to source locally and shipping costs from UK have gone through the roof, plus the two week wait for it to arrive…. So I have resorted in some instances to making up new accessories for my Myford but using UNF threads, but making my own hex nuts with UNF threads but BS sized hexagons so I only have to use the one set of spanners to work on the Myford. Some poor future owner will have fun trying to work that one out!

ME AGAIN, for those who need them I have a stock of 1/4" BSF bolts 3/4" up to 4" long ! Noel.

Perhaps it would be possible to convert your UNC hole to a standard, readily available and cheap metric bolt using a common Helicoil? That would cost the same as the UNC tap, but then be useful elsewhere.

Americans love labelling things with weird designations, like number and letter drills, #threads, dash hose sizes, schedule pipe etc. Metric does this better; you don't buy 18gauge steel, but order 1.2mm thickness so that no tables or other arcane knowledge is required. Although I do think the earlier suggestion of using MC, MS and MF for common metric threads should have been the standard.