BSW sockets and spanners are the correct "size" for Whitworth (BSW) and BSF bolts and nuts. BSW and BSF are essentially read as a dual size yet are the exact same width "across the flats". A BSW marked size is generally 1/16 smaller than a marked BSF size yet they are the same.
Here's a useless titbit of information. 5/32" BSW threads have been the standard Meccano thread for many years and it is still the thread in use by the French Meccano Company!
Old cars and bikes manufactured before the late 1960’s tend to use fixings that require an "inch" or "imperial" based tools. As a result, BSW (Whitworth), BSF, AF (Across Flats), or even BA (British Association) sockets and spanners are collectively referred to as "inch" or "imperial" and they are the correct sizing of tools often used on classic and American cars and bikes. Although some later, more specialised vehicle models continued to use BSW (Whitworth) for many years after other vehicles had stopped their configuration.
That said, it is important to note many vehicles manufactured in the period reaching the mid to late 1960’s still used bolts and nuts that required BSW sockets or BSW spanners including many English built classic cars and bikes.
So there's no real clear rule except
If a Metric size doesn't fit well, it's either AF, BSW or BA.
If an AF size doesn't fit well, it's either BSW, BA or Metric.
If a BSW size doesn't fit well, it's either BA, Metric or AF.
If a BA size doesn't fit well, it's either a Metric, BSW or AF.
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