[rotorwrench]

Screw threads have evolved a lot over the years. Unified National Course and Fine are pretty well know but there is also a Unified National Special or UNS thread pitch specification that has stumped more than a few old prewar Harley Davidson enthusiasts. The special pitches they used were half way between UNC and UNF threads. I guess they wanted the exclusive rights to the HD hardware. Finding the taps & dies for these is difficult but not impossible. I have the most common sizes. Another odd one was the British Standard Cycle thread. No matter how big the bolt shank is, they all had 26-TPI threads.