"Cycle" threads are an abomination. While at N-V, I tried very hard to get all fasteners standardized to US threads, or at least all UNF and UNC, but without success. To a large extent, that was blocked by suppliers who used the other standards. All too often we heard the argument "you're such a small account, why would we change?" That was very much the case with Lucas.
Cycle threads are a result of cheapness and greed. The bicycle industry standardized on 26 threads per inch, independent of fastener diameter, firstly to save money on thread-cutting machinery and secondly to make money selling these screw-ball fasteners to bike owners, because they weren't readily available from sources other than bicycle shops.
Of course there are also British Association (BA) threaded fasteners. I think it's interesting that so many BA threads are 25.4 TPI. Isn't that 10 threads per centimetre? If you look at the screw diameters in the BA series, I think you'll find that they're all round numbers of millimetres. It was a sneaky way of intrducing metric fasteners under a false banner.
Having worked in the fastener group at Boeing for a while, I concluded that the US fastener industry has developed a lot of very high tech stuff that the European industry still hasn't caught on to, but for the road vehicle environment, the old UK standards and the various kluges and add-ons seem to work OK - they're just a real PITA to try and identify when you're doing a rebuild.
My recommendation is that, if it's a nut and bolt, go for US standards. If it screws into a threaded hole, you're stuck with whatever the hole's thread is.
BTW, as defined in aerospace standards, a properly assembled bolted joint has no threads inside the holes in the components being joined. The threaded section of the bolt should end approximately 1/32" above the face of the joint and a 1/8" washer should be used between the nut and the joint face to allow the bolt to be properly tightened.