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BSB vs Cycle Threads

Error in ME Article

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houstonceng10/02/2018 17:02:50
26 forum posts

I've just read the latest episode (Part 7 - in Issue 4580) in "Lathes and More for Beginners" by Graham Sadler. Although he should be congratulated for attempting an "All you need to know, but were too afraid to ask" type series, he is guilty of propagating an error that could lead to beginners buying the wrong taps and dies.

​When talking about the various types used by Model Engineers, he states "BSB - British Standard Brass is 26 TPI (teeth per inch) also known as cycle thread".

​Whereas he is totally correct that both BSB and Cycle Threads are 26TPI, they are not the same. BSB is a Whitworth form of 55 degree included angle, (as are the 32, 40 and 60TPI ME Threads, BSF and BSW), however, cycle threads are 60 degree included angle.





Edited By houstonceng on 10/02/2018 17:05:09

Marcus Bowman10/02/2018 20:28:04
162 forum posts

British Standard Cycle threads use a 60 degree threadform, with a shape which visually resembles the Whitworth form. The 60 degrees makes the thread 'flatter', but the crests are rounded, like the Whitform threads, although the radius is different. The height of the BS Cycle thread from crest to crest is 0.5376p as opposed to the Whitworth at 0.640327p.The radius of the crest is p/6 (= 0.167p), as opposed to the Whitworth crest radius of 0.137329p.

So; BS Cycle threads are not Whitworth threads.

BUT, the author may have taken account of the fact that BS 811 : 1950 (still the current standard) provides the following note:

'It is customary practice in the cycle industry to use a 20tpi series of Whitworth form threads as an alternative to the cycle form thread series...'

That's not the same as saying that the BSCycle threads are Whitworth; and the practice seems only to apply to 20tpi threads.

Although, in general, BSCycle threads are 26tpi, the smaller diameters (1/8, 5/32 and 3/16 inch) are not. 1/8" is 40 tpi, while 5/32 and 3/16" are 32tpi. It would not surprise me to learn that the 1/8" BSCycle thread is sometimes used in place of the 1/8 x 40tpi imperial Model Engineer thread, especially where the focus is on the diameter and 40tpi specification in a mixed bunch of older taps or dies, but it is not the same thread, and should not be a fit for the proper Whitworth form ME thread. Mating the two will result in damage to the thread flanks even in a short hole.


Hopper11/02/2018 02:52:27
3979 forum posts
85 photos

Meanwhile, in the real world, 55 degree and 60 degree threads are screwed together all the time without problem. EG the UNC and BSW threads that share a common pitch. So much so that currently in Australia if you buy BSW bolts and nuts up to 1/2" diameter, you will receive UNC items labeled as BSW. The head sizes give them away, which is a real nuisance when restoring old machines and trying to use only one set of spanners!

Correctly made threads will have some clearance between the mating parts, which is generally enough to accommodate the slight difference in flank angles, ie 2.5 degrees per flank. Crest radiuses don't even come into it in the real world. The male thread should as a matter of course be made a bit undersized to ensure there is no crest-to-root contact.

Of course, if making a CEI or BSB thread from scratch, it's best to use the correct angle, but not usually super critical if you for some reason can't.

'Tis surprising that the CEI fellows chose 60 degrees when they lived in a 55 degree world though, and I am sure our author is not the first to be caught out by that detail.

Marcus Bowman11/02/2018 07:37:57
162 forum posts

Yes; the real world often deviates from tight standards, for practical reasons. It seems to be common practice in the USA to truncate the crests of the Whitworth threadform, presumably to remove the need to accurately form the crests of the male threads. That removes the problem of interference between Whitform crests and metric/UN roots, British Standards defines the dimensions of truncated Whitworth threads, saying that 'the rounded crests at the major diameter of the external thread and at the minor diameter of the internal thread [are] removed at their junctions with the flanks'. I did read somewhere that the original reason for truncating the Whitworth crests was because the standard of gauge-making in the USA, in the early days, lagged behind the accuracy of British gauge-making, so truncating the crests removed that problem from manufacturers. Truncating certainly makes a lot of sense. The clearance between mating threads depends on the tolerance class of thread (class of fit) but most commonly available threads are of the 'normal' class i.e. the sloppiest fit, so its no surprise that some 55 and 60 degree threads of the same pitch can be mixed. It's a bit like mixing BSW and BSCycle if they both have have 26tpi.


colin wilkinson11/02/2018 08:11:24
58 forum posts

Sorry if I am missing something, my model engineering experience is zero, but as a motorcycle bodger of many years using Cycle thread 1/4 BSF has 26tpi not BSW. Common to find BSF fitted as replacements as normal suppliers don't carry BSC. Some bikes like the BSA Bantam have a mix of BSW, BSF, BA, UNF, and BSC originally as it was produced over many years. My pet hate is restoring a bike to find metric fasteners and even worse, threads re-tapped metric. Colin

Marcus Bowman11/02/2018 09:14:03
162 forum posts

As a motorcycle enthusiast, I have had many British bikes,and I certainly agree about the lack of sympathy in any restorer who attempts to convert British threads to metric simply to avoid finding or making a proper replacement Whitworth or BSF thread. Very nasty.

BSF threads are the same threadform as Whitworth threads but have finer pitches. BSF threads are specified in the same British Standards document as BSW threads, as the fine-pitch version of the Whitworth series. Hence the term 'Whitform' which covers all the thread series which have the same thread outline (55 degrees, rounded crests, tolerance classes, etc). You are right: 1/4inch BFS has 26tpi, whereas 1/4inch BSW has 20tpi. BSF extends the Whitform series by providing finer pitches for the corresponding diameters, compared to BSW.

I always felt the BSF fasteners where a more elegant thread series than the visually coarse BSW bolts. But that's just me. BSW is probably more appropriate in aluminium or cast iron, while BSF would be my preferred option for steel.


Lynne11/02/2018 09:25:13
73 forum posts
26 photos

Colin, Ihave sent you private message, re bike restoration. Lynne

vintagengineer11/02/2018 11:07:58
468 forum posts
6 photos

I Know how you feel! I rebuilt a French Amilcar engine for customer.The engine had been in the UK since before the war and had been worked on numerous times by bodgers. The threads are supposed to be French metric and I had BSF, Whit, UNC, UNF and ISO Metric!

Some I could helicoil but most I had to fit threaded bushes.

Posted by colin wilkinson on 11/02/2018 08:11:24:

Sorry if I am missing something, my model engineering experience is zero, but as a motorcycle bodger of many years using Cycle thread 1/4 BSF has 26tpi not BSW. Common to find BSF fitted as replacements as normal suppliers don't carry BSC. Some bikes like the BSA Bantam have a mix of BSW, BSF, BA, UNF, and BSC originally as it was produced over many years. My pet hate is restoring a bike to find metric fasteners and even worse, threads re-tapped metric. Colin

houstonceng12/02/2018 16:41:51
26 forum posts

Hi Colin

​Yes 1/4" BSF 26TPI is the same as the 26TPI Model Engineer thread of the same size, however, the TPI on BSF and BSW varies with the size of the thread - as does the pitch in mm of metric bolts. BS Brass and BS Cycle threads are 26TPI for a range of diameters, but as the former is a Whitworth form with 55 degree included angle and the latter is 60 degree, they are not compatible.

​Despite what some people say, 55 degree and 60 degree threads do not mate well and forcing a 60 degree into a 55 often damages one of the threads.


Mark Rand12/02/2018 22:43:11
849 forum posts

Curiously, camera tripod threads were historically (and should still be!) 1/4" and 3/8" BSW. But in about 1972 the ISO decreed that they were 1/4" and 3/8" UNC, thereby destroying another bastion of civilization. Luckily, most camera and tripod mounting threads are so sloppy that it probably wouldn't make much difference if they were BA 47.5° or ACME 29° thread forms...


Mea culpa:- I once had to use a 3/8x19 BSPT fitting in a 3/8x18NPT socket, at 2000psi! Why the H3ll British Steel at Port Talbot had bl**dy American threads on that instrumentation pipework, I never discovered. Luckily, It survived the four weeks needed for the work. To add to the weirdness, the stainless steel instrument pipework was all etched/printed with labels declaring it as 12.7mm OD crying.

Edited By Mark Rand on 12/02/2018 22:51:27

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