Although written for a British motorcyle audience, this also applys to many earlier British cars. It was reproduced on the British Cars list with permission from the author Charles Falco.
First, a tiny bit of history. In the 19th Century every British factory which needed to bolt something to something else devised their own fasteners to do it. Clearly, this caused all sorts of compatibility problems. So, along came Mr. Whitworth (I forget his first name right now) who invented a standardized system of coarse threads (with 55 degree thread angle and rounded roots and crests).
This standardization was a Good Thing. Along with his threads came heads for the bolts that were based on the length *along* the side of one flat, rather than across the flats. Hence, there is no simple fractional number for the length across the flats, which is why your American wrenches don't fit. The fractional number on your English wrenches refers to the diameter of the bolt (which is 1/4", 3/8" etc. just like in the U.S.); not to the distance across the flats (which ends up being various weird dimensions). Some years later the Brits decided they needed a finer pitch for some applications, so another thread series was introduced (same 55 degrees). They also decided that the heads were too big for the bolts, so for most applications they switched to using the next size smaller heads. Because of this, and to add one more bit of confusion to life, one manufacturer will mark a particular wrench (spanner) "3/8BS," while a different manufacturer will mark the same sized wrench "7/16W." They fit the same diameter bolt.
The first thing any fledgling Brit biker learns is that his (or her) motorcycle has "Whitworth bolts." They think this is interesting, buy a set of "Whitworth wrenches," discover these wrenches fit their bolts, and believe they now know everything they need to know about British fasteners. Unfortunately, at this point they know only enough to make themselves dangerous. Instead, what they *should* have said to themselves is "Ohmygod, what other weird and incomprehensible things have the Brits done to the fasteners on my machine?" The answer to this question is:
60 degree thread angle, rather than the 55 degree of BSW and BSF. For sizes from 1/4" through 1/2" by far the most common are 26 tpi, although 24 tpi appear as well. *Most*, but by no means all, fasteners on post-War BSA's (through the late '60's, when it got more complicated) were CEI. Although the thread form and pitch is different, the head sizes on CEI-threaded fasteners use the same wrenches as BSW/BSF.
So, let's get back to the question someone asked a few days ago about buying a set of "Whitworth" taps and dies to last him the rest of his Brit biking life. The first thing I'd say is that if you wants it to last, be sure you buy a HSS set (rather than carbon steel), even though it will be at least 2x the price. Then, in order of overall usefulness, I believe you'll find in first place it will be a CEI set, then BSF, then BA. Unless/until you really get deeply involved, make do with UNC to "simulate" BSW (although you'll want to buy an individual 1/2"-12 at some point to supplement the UNC set). Thus, if you follow my recommendation, the very *last* thing you'll want to buy for working on your British motorcycle is a "Whitworth" set (I have one, by the way, but only got it after I'd had CEI, BSF etc. for a long time).