There seems to be a bit of confusion, here.....Whitworth was a coarse thread, the first standardised one in the world,
afaik.
BSF (british standard fine) was the engineering alternative to the "agricultural " BSW the spanners were all dual-marked. IE 1/4W-5/16bsf the other end being 3/16-1/4 or 5/16-3/8 Thus, a set would always have 2 ends the same size to tighten /loosen a nut and bolt
Then, after the war, around the mid-1950's we adopted the American Unified system....UNC and UNF They were different, in that a1/4 bolt had the same head-size (measured A/F - across the flats0 IIrc,a1/2 AF fits a 1/4 bolt or nut....the smaller sizes are almost identical to the Whit/BSF range and can, at a pinch, interchange....above 1/4" the pitches diverge and , IIrc, a 5/16 unified will foul a BS counterpart after about 1 1/2 turns...Unified can be identified by a ring stamped in one end of the nut,or a chain of small rings down one flat. the bolt has a circular depression un the head, ir 3 equally-spaced radial lines.
British Association was a standard electrical thread....not only beloved of the prince of darkness ( parody on the Joseph Lucas slogan "King of the road"
) All domestic electrical fittings were standardised as to spacing and thread, thus a single switch had 2 x 2BA screws at, IIRC 2 3/8"centres.......then we went Metric the imperial centres were retained , but we now used 3.5 MM screws, which
looked identical but, of course, jammed solid after ~2 turns......re-tapping the sunken box was one option, re-using the old BA screws was another. Istill come across BA screws in boxes!
When the tool business went a bit quiet (coinciding with the influx of foreign cars, ) we went metric in the engineering-trades. Again, a few thou difference on some spanner-sixes....1/2 AF = a very tight fit on a rusted/worn 13MM head...14 MM is tight on a 9/16
Metric coarse and UNF are fairly similar to look at, but not at all compatible...Metric Fine is a bit unusual....again as referred to by
A500 man there are some anomalous sizes where 2 different pitches are used for a standard diameter....also, the head-size does not necessarily always bear the same relationship to the actual bolt diameter..
Metric spanners are quoted AF...so that's the head-size across flats, not the shank diameter.
A real oddball....British Cycle thread!....that's why those pressed-tin multi-hole bike spanners never fitted anything else! Threads were very fine pitch and shallow, for the diameter.
Ireally must get out more!
