Tell me what you want to know and I’ll do my best!
It’s reasonably simple. The size on the socket is the size of the bolt, not the size of the bolt head. So, while you need to know that a 1/4 UNF bolt has a 7/16 SAE bolt head. In the British system, a 1/4 BSF bolt uses a 1/4 BSF wrench!
There are two thread forms, Whitworth, now technically BSW (British Standard Whitworth) and BSF (British Standard Fine). The former are coarse threads, optimised for cast iron, the latter are fine threads, for steel. Very similar to UNC or UNF.
BSW threads originally used relatively large bolt heads, BSF slightly smaller. This is why many wrenches have both sizes marked e.g. 3/16 BSW and 1/4 BSF. Since the war, the large bolt heads were dropped, so now many wrenches are just marked with one size, e.g. 1/4 BS.
BSW and BSF threads were used universally until ‘unification’ (when the U.K. Canada and U.S. adopted the same system). The aerospace industry was the first to go Unified, followed by the motor industry (in ‘58 I think). So if you have one of these products that post date this, use your SAE wrenches.
Some industries never really adopted Unified threads at all. The bike industry for one. The machine tool industry was another. Some machines (Colchester lathes) will have BS bolts on older machines, Unified on newer. Some makers never made the switch, and are producing stuff with BS bolts to this day. Many went from BS to metric and never used unified.
So, which sizes? On most cars and bikes you can do probably 80% of what you need with 1/4, 5/16 and 3/8 BS wrenches. If you start playing with things like hub nuts you will need bigger, and likewise on machine tools. If your Mustang uses 1/2 and 9/16 wrenches, these will be for 5/16 and 3/8 bolts. The BS equivalent would simply be 5/16 and 3/8.
You might also need some BA sizes. BA are the British equivalent of the SAE ‘numbered’ series. These are common because the U.K. never adopted the ‘numbered’ series, so BA continued till ISO metric was adopted, if it ever was. Some industries never did. Koken offer some very nice BA sockets, in both regular and deep.
Personally, I would hang on to any BS tools, certainly in the popular sizes. I have a ‘road box’ that I do a lot of work from, and I always keep 1/4, 5/16 and 3/8 BS wrenches in there, together with 2 and 4 BA.
If I’m setting up machines, I take a full set of BS box end wrenches and open enders with me, as well as 3/8 drive sockets. In my ‘electrical’ box, I keep 0,2,4 and 6 BA for electrical terminals.
