BSPT, UNC, UNF, BA...................
BA is metric. It just does not know it.
https://en.wikipedia.org/wiki/British_Association_screw_threads
Nope, not if you read further:
The thread angle is different from that used by Whitworth (55°), US Unified threads (60°) and ISO Metric (60°) so BA fasteners are not properly interchangeable with Whitworth or metric ones even when the pitch and diameter are similar enough that they can be screwed together (e.g., although 0BA appears similar to M6×1mm, the male and female surfaces do not mate properly for bearing a load). Standard BA sizes are defined between 0BA and 22BA. The even sizes are much more common and the very small sizes are not used very often and other standards tend to be used
While M6x1 is very close it's the only size when I look through my thread chart.
Most other threads are 55 or 60o whereas BA is 47.5°
Both Bicycle Thread and BSW (to name a pair of outliers in terms of pitch coarse-ness) are imperial, but they do not mate. So why is it suddenly a requirement that all metric threads of a given major diameter mate? There are, to take a common example, three different pitches on M10, the common coarse being 1,5mm, and then 1,25 and 1 mm also exist. Thus, the only reasonable definition whether a thread is metric or not is if its dimensions are originally defined in SI units. (or more correctly, their ancestors, since there was no SI system in the late 19th century; metric was governed by C.F. Gauss' CGS system, to be replaced by the MKS system starting in 1889. SI was adopted in 1960. Most people probably didn't care but simply used centimetres etc.)
Of course, one can argue that B.A. thread is not ISO Metric, and that would be true, because there is no ISO standard for B.A, except, perhaps by transition of a British Standard. I can't tell, because ISO is one of those Back-asswards organisations, like IEEE, that want money for standards text. (IETF way being the only sensible way of doing it, IMNSHO), but, the following quote from the B.A. commission No 2 Report sort of sums it up nicely:
4. The manner in which the series of screws adopted lately by Swiss manufacturers is correlated has been sufficiently explained in the previous report, and very full explanations are given in the two original pamphlets to which reference is there made.² The diameter (D) is related to the pitch (P) by the formula D = 6P6/5, (1) all measurements being in millimetres, and P having successively the values 1 (or 0.90) mm.; 0.9¹ mm.; 0.9² mm.; 0.93 mm .… 0.9n mm.
(my boldfacing)
/Måns, has a full set of open-jaw spanners and tube box spanners for BA.
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