Long before WWII, there was some controversy in the USA as to which thread form was better for certain applications. Where this seemed to come into play was with staybolt threads for boilers, mostly on steam locomotives. For some reason, certain railroad mechanical departments in the USA would specify that the boiler staybolts be made with Whitworth threads. The rest of the boiler would have National Taper Pipe threads and what was to become the Unified National Form threads in the usual standard threads for bolting.
At the hydroelectric plant where I've worked these past 24 years, we have quite the mix of systems of measurement and screw threads. Our big pump-turbines were made in 1970-72 by Hitachi, in Japan. At the time, the bid specs called for dimensions in inches and US threads. For the most part, Hitachi complied. We have studbolts with threads as much as 8" in diameter on some of the studbolts, cut to various pitches but using the Unified National Form. On pipe threads, Hitachi seemed to favor British taper pipe and British Straight Pipe using copper crush gaskets. Here and there, on stuff made by subcontractors for Hitachi (such as valves, lube oil pumps, gauges, pressure switches, all made originally by Japanese firms), we have metric threads and all kinds of adaptors. Add to this that we can find ourselves with metric sized pipe and tubing or inch sized tubing and regular "iron pipe" sizes- all on the same units.
The original air circuit breakers in the high voltage switchyard came from Switzerland, as did the high pressure air compressors for them. Metric fasteners, British pipe threads both straight and tapered, depending on type of fitting. The original generator step up transformers were made either in Italy, Germany, or Switzerland depending on which transformer- some by Brown Boveri, some by ASEA, some by Alstom. Same story- you go to work on a conservator tank or oil cooler on a transformer made in Europe, and one transformer will have metric fasteners, another will have Unified National Form threads and use regular "US" bolting. Pipe flanges on this stuff can drive a person up a wall, as some will be DIN (German standards), some will be ANSI, and some will be JIC (Japanese) facing/bolting patterns. Down in the plant, on the Hitachi turbines and generators, we run into flared high pressure tubing that may use JIC (Japanese standard) or SAE, no rhyme nor reason or consistency. The result is that even though we've owned and operated this plant for over 40 years, we still measure and verify fittings, bolting, tubing and pipe sizes, flanges, and on it goes. Hitachi used miles of O rings on all sorts of joints, from tiny O rings to using O ring material off of spools to seal split joints on the turbine parts. Naturally, these details on the drawings are often kind of sketchy, with nomenclature half in English, half in Japanese, and the smaller nominal dimensions are often either not there, or hard to read. So, we mike the O ring materials, measure the grooves the O ring material goes into, and I often run a separate calculation to be sure we have the right size O ring material- too small and it won't seal, too large and it will extrude at the joint line.
We live will very nearly every imaginable type of screw thread, pipe thread, and flange or flare fitting system in our plant. Sometimes, we have to make adaptors to go from metric pipe to US iron pipe sizes, which is where having our in house shop comes in handy. Sometimes, I will detail a "compromise flange", which may be bored as a slip on flange to fit on some odd pipe size, and then faced and drilled for something else again (like DIN or JIC instead of ANSI, or faced and drilled for ANSI, but bored to fit on a metric pipe). Other times, it may be a socket weld adaptor that we machine from bar stock to couple two different systems of pipe.
Not too much surprises me anymore. I know that at the dawn of the automotive age, metric spark plug threads became the standard. Very early spark plugs for some of the hit-n-miss engines or the early auto engines used NPT threads, but somewhere along the way, the world seemed to settle on metric spark plug threads. A car, truck, engine, motorcycle or aircraft engine built in the USA could have all the dimensions in inches and use Unified National Form threads in standard threads (UNC or UNF/SAE), but the sparkplug thread was always the exception, being metric. I never did learn the reason for this. My own guess is a lot of the early successful cars were coming out of Europe, and perhaps the early ignition systems came from Europe as well (Bosch, from Germany, perhaps Eisemann and Splitdorf for magnetos, coming from Germany & Scintilla from Switzerland). Maybe someone can shed more light on this little anomaly.
