I don't know if we should make a new topic, we can always split it later if needed I guess,,,
Metric with unc; bsw with unc
BSW was the first real standard for threads,,, British
UNC was based on Inch as well but American US - Metic was based on Meters, European
Here goes quickly some of the differences as to understand the basis for some differences:
First threads where custom made locally, obviously this created some issues when replacements where needed. So entry of Standards BS US DIN ISO JIS etc. Also some still are by company, country, region or global...
Our company, ship engines, use selected few thread sizes, they come in a Standards Book, to obtain as few variants as possible, less stock needed and less spares needed in design, models in 3D drawings, in the factory, in stock all over world and on the ship, same goes for the tool sizes etc. since our engines are build in Asia we use a base of standards that is a mix of JIS and ISO, old DS Danish Standards and even ship standards,,,
The use of the actual threads demand even further differences, like the ultimate in strength are trapezoidal threads they have a very large area similar to with the plastic, but if used in metal naturally the screw can also have the trapezoidal shape.
We use a special variant of trapezoidal threads since we tighten the long screws, studs, with hydraulic jacks, so the nut must be able to be turned only using hand tools while the stud is stretched to obtain the desired pre-tension - Therefore we have a company standard for that thread, and the allowed variant sizes.
The car industry has used Inch sizes and some still do, they have a more coarse thread size generally, preferred since easy disassembly many years later, rust etc. is more easy but also re-use of assembly tools as well as spares like above is a huge consideration,,,
For Aircraft you need low weight but high strength so the design and solution is a shorter bolt and nut with finer threads, less steep angle to get the number of threads engaged high to carry the load,,,
