I composed this a few years ago, it seems that it might be of some use here for those who might not have a perfect grasp over what can be, admittedly a nightmare to understand>
Historically, there can be few things that have created more confusion for the classic British bike enthusiast, especially those who were born in the metric age and those not from the UK, than the difficulty of translating the intricacies of obsolete British Standard thread systems and by extension, wrench sizes (spanners) into language comprehensible to the layman. Because of this, misapprehensions have been perpetuated. Terms like ‘whitworth’, ‘Imperial’, ‘BS’ and so on are applied casually to things that they do not belong to. This leads to further confusion.
If there is any aspect of British engineering which causes more hair to be pulled out in frustration, I'm not sure I know what it is...
When referring to wrenches and threads, ‘Imperial’ means BOTH what some refer to as American inch, AND British standard. They are of course utterly different, to make things as confusing as possible, but they are both Imperial systems of measurement. (EDIT: or are derivatives of the Imperial system)..
American inch wrenches are measured "Across the flats (A/F)" That is, the "size" of the wrench is measured according to the hex on the fastener, a 1/2" A/F wrench tightens or loosens a fastener with a 1/2" hex. These are the wrenches and fasteners used on Chevys, Fords and Chryslers for generations...
…Older American wrenches using the SAE system were also marked according to bore size; however, but we’ll just leave that there to save complicating matters further.
British Imperial fasteners are measured according to the BORE of the fastener itself, so a 1/2 BS wrench fits a fastener with a 1/2" bore.... In other words, when you look at the wrench, you might say to yourself "Self, there's no way that's a 1/2 wrench"... but it is of course, because it turns a 1/2" diameter bolt...
Now hang in there, stay with me, cos this is gonna get outrageously ridiculous. We're even going to take a pause, Put the tea on and make sure you have an ample supply of crumpets for this one...so you can gather your wits, you're going to need them for this next bit. BS vs. BSW vs. BSF…
...In the beginning, there were no standard threads anywhere. If someone wanted to screw something to something else, they cut their own threads.
It’s tricky to conduct a successful industrial revolution when you don’t have standard thread forms.
A gentleman named James Whitworth came up with a standard system of threads. He designed the famous 55 degree pitch thread in a number of sizes from small to large, and then proceeded to pick standard sized hexes for each given bore of fastener cut with his thread. He picked sizes for the wrenches he wanted to use to turn his fasteners and his "hex sizes" do follow some arcane mechanical engineering principle, I am not aware of what that principle is, but suffice to say , they are what they are. (the nice thing, is that for the range of fasteners, there are fewer wrenches needed.)
James Whitworth's first coarse thread series became known as BSW (British Standard Whitworth) the very first standardized thread form in the world, hence why he stuck his name on it. It is NOT a wrench size, but a THREAD.
Mr. Whitworth's system did well for around fifty years in the early Industrial Revolution, but after fifty years had passed, the engineering wonks decided that an additional FINE series of threads was required... Hence BSF (British Standard Fine), another series of threads of finer pitch.... And here is the fly in the ointment.
They decided that Mr. Whitworth had been too generous with his hex sizes, and that a given hex could turn a fastener with a larger bore. This decision was taken due to metallurgy... Steel was getting a lot stronger due to advances in smelting and steelmaking..
Anyways, what these boffins did was curse the world. Because of their decision, a given wrench may turn a coarse threaded fastener of one size, or a fine threaded fastener of 1/16" greater bore....
example: 1/2BSW - 9/16BS * .
So in this case, the wrench above turns a coarse threaded fastener of 1/2" nominal bore OR a fine pitched fastener of 9/16" nominal bore...
This is why wrenches are often encountered with two apparent sizes stamped on them. (5/16W-3/8BSF)
to add a final poisoned nail to the agony, when you get to quite large sizes, the difference goes up from 1/16" to 1/8" difference. So you have big spanners for buses marked 3/4W – 7/8BS - Madness.
During the war, the sizes were standardized in that newer fasteners were made with both fine and coarse threaded bolts and nuts had the same hex...
* to add confusion, often the "F" is omitted, Snap-On’s wrench set, for example, omits the "F" thus: 7/16BS, and doesn’t list the ‘W’ at all.
So, to clear up this section, Whitworth isn’t a wrench size but a thread.
American or Unified threads (UNF, UNC) and associated A/F tools are “Imperial”.
British Standard threads (BSW, BSF, BSCy) and tooling, are also “Imperial”.
BSW is a coarse pitch thread series.
BSF (or BS) is a fine pitch thread series. (as an aside, it is rare in British bikes but occasionally encountered on BSAs)
The same wrenches turn both types of fastener, but the fine threaded fastener will have a smaller hex.
Oh, sorry, sit down, we’re not done.
BSCy. British Standard Cycle Thread, or CEI (Cycle Engineers Institute)
Adding to the confusion, and funnily enough the most common thread system you are likely to encounter if you own a machine dated before around 1966 , is BSCy, or Cycle thread. It is a fine (increasingly so as the bore size rises) pitch thread, British Standard, but not whitworth form, 60 degree pitch rather than 55 degree.
Cycle thread was designed for use in bicycles, to combat loosening due to vibration. It was deemed ideal for use in motorcycles, and appears in almost all marques up until the late sixties, and there are internal BSCy threads on Triumphs and Nortons into the seventies.
This is an interesting thread system due to the use of consistent thread counts for numerous fastener bore sizes. A ¼’ bore fastener has 26 threads per inch (tpi), and so do all the sizes up to ½” bore. There is crossover around that size to 20 tpi, so there is a 7/16x26tpi thread and a 7/16x20tpi thread and the same with the ½” bore. The main nuts and bolts with this thread on British bikes will be 26tpi, except for gearbox and engine shafts and axles, which often have 20tpi. (Very small BSCy fasteners have 32 tpi but are very rarely seen on motorbikes)
Note: BSCy. (Cycle thread) fasteners are turned with the same wrenches used for BSW and BSF.
Taps and dies for all of these threads are available, old stock BSW and BSF taps and dies are available though good sources in the UK, but BSCy is tricky to find, and only from Asian sources now.
Horrifically, there is also the small BA (British Association) series, found in Lucas stuff, electrical things mainly, badge screws and the like, and BSP, British Standard Pipe, for oil lines… and, if you own an old Panther, Admiralty thread. I’ll tell you if you ask nicely.
Note: BA wrenches are different, a small series of 8 or so usually, British bike guys can get away with the four or five biggest ones. Sizes from 0-12, 12 being smallest.
Final edit: Newcomers to the hobby should be aware that many bikes from the late sixties and early seventies are especially confusing due to the introduction, for the benefit of the US market, of American inch hardware (A/F) and UNF and UNC threads initially on the exterior only. The whole thing was introduced piecemeal over a period of several years. Any BSA, Triumph or Norton, commencing in around 1968 and running right through till 1975, can be expected to have a mixture of hardware and threads in sundry places. Many UNF nuts and bolts on Nortons are stamped with a line of circles on the flats of the hex indicating UNF.
Hope this helps someone, took me a while to wrap my head around it when I first started.
Make mine an ESB.