If you already have a thread gauge, or good eyes and a measuring stick that's sensitive, you'll find that Cycle in all sizes up to very big ones is 26 threads per inch. Some big sizes had a 20TPI alternative. BSF is only 26 at 1/4 in diameter. Where nuts and bolts go through plain holes, anything could have happened over time - but where screws thread directly into iron or alloy, you'll find that most things are Cycle or BSF into iron, and Whitworth into aluminium. Spanners are the same for all although the sizings are expressed differently for WW. A fair few WW threads are the same tpi as UNC, and often one finds UNC fasteners where really it should be a Whitworth. UNC and UNF were the preferred broad equivalents of Whitworth and BSF in the USA, and became the standard in the UK in the 60s, before everything gradually migrated to metric. Many '60s cars were a confusing mixture.
You may find some UNF threads on some frame parts - I have anyway, whether made that way or messed with/replaced out of period I don't know. The common bolt/screw diameters there are 5/16ths and 3/8ths (all imperial), which take spanners marked 1/2 and 9/16ths 'AF' respectively (across the flats). These are both 24 tpi and can be confused with Cycle to the naked eye as they are fairly fine for their diameter. You may also meet slightly larger UNF sizes which require 5/8ths, 11/16ths or even 3/4 AF spanners. All are finer than their BSF equivalent.
The only other fasteners that can cause you grief are the electrical ones, which are largely BA, British Association. A mix of small metric and AF tools will undo most of them at a pinch, especially 4BA, which is near as dammit 1/4AF. This one matters, cos that's what the bolt that holds the contact breaker assembly in the mag is. Other mag and dynamo bits are also BA. BA tools are widely available in the UK and a worthwhile investment in my view.
It is very unlikely that you'll find any metric fine threads on your bike, but not impossible I suppose.
The 1/4 cycle/BSF similarity often causes confusion as the only difference is in the thread angle; by the same token 6 by 1mm, a very common modern size, is the same size as 0BA apart from the thread angle.
Thread angles are, for all sizes, 60 degrees on metric and Cycle, 47.5 on BA, and 55 degrees on all WW and BSF. UNF and UNC (its coarse WW-like counterpart) are also 60 degrees.
To distinguish between all these things, you need thread gauges as derek says. As a quick aid, it's useful to have a handful of nuts and bolts of known sizes, which you can use as test parts to see what fits what. In most cases, the wrong thing won't go on - but proviso re 1/4 Cycle/BSF and several WW /UNC sizes). After a bit, you'll just look at a piece and know what it is without having to think about it.
None of the above necessarily applies to what you'll find inside a gearbox . . . there be unusual sizings in them sometimes.
Agree totally about Tracy Tools, excellent source of affordable threading tackle and very helpful info (and nice people).
Good luck!
