A must read backgrounder first.

Then the brilliant Backgrounder from gREEgg's below :

Regarding thread forms, I note that Bob Cram sent a URL with some info. The short answer is that as England became industrialized, each industry developed what it thought would be the thread forms to best suit its needs. And of course, these threads were not adopted by North American industry.

You are right that Whitworth is analogous to our UNC, and BSF is like our UNF. There is also a BA thread, which is mainly found in smaller electrical fasteners, and CEI, which was defined by the Cycle Engineers Institute. CEI is unusual, because it specifies a 60 degree Vee thread, but with a constant 26 tpi thread for all sizes from 3/16 to 9/16" .

To be 100% accurate, I should add that there are a couple of the smaller sizes with a finer thread, but I have not committed those to memory. Our bikes are built with a mish mash of all these threads, and I've noticed that the thread pitch used say, for the rear axle nut changed from year to year. (bet no one can guess how I found that one out ...)

There are a few instances in which our SAE fasteners can be forced to mate with the British ones, because teh threads per inch are the same ... 1/4 x 20 is one that comes to mind.

However, even then the fit is not great, because SAE threads are Vee shaped* with a 60 degree angle, while BSF and Whitworth are cut with 55 degree threads having rounded roots and crests. All else being equal, this makes for a fastener with superior accuracy and strength, at the cost of a higher manufacturing cost. (*Most SAE threads are now flat top and bottom to improve fatigue strength and reduce manufacturing cost).

You'll notice that many of your bike's original fasteners were cut on automatic screw machines, while most modern fasteners are rolled. While accuracy is OK, this allows low-cost mass production, and the use of very strong steel alloys, which could not be economically screw-cut the old fashioned way.

All this of course became moot as England exported more and more machines to North America. First, the automobile industry converted to SAE fasteners by about 1950, and the motorcycle industry converted about 1970.

In terms of ordering new fasteners, I've found that many of the replacement ones are of pretty poor quality. Therefore, I spend a lot of time re-claiming original fasteners where possible. I build up worn or hacked areas with weld, machining back to size.

It pays to have a screw cutting lathe to rough out the thread, and size them with the proper British die. Some applications, such as with through studs, it is possible to replace with an entire new SAE part.

In other cases, you may be able to cut down a longer stud, and re-thread the new end with an SAE thread. As with so many things in restoration, ingenuity is the call of the day.

I've only scratched the surface, but I hope this has been of interest

gREgg