Early on in my hose career, I had a friend ask me to go pull a 10MP (male pipe) fitting for him. After quite a while of meaningless searching, I was let in on the reality that pipe thread does not come in a -10 (5/8”) size. I came back to the guy slightly defeated. I asked, “Why doesn’t pipe come in 5/8” when everything else has a size 10?” It made no sense to me. The answer I got back was just as frustrating as the question itself…. “It just doesn’t, They never made it, I don’t know.” That was the extent of the answers that I got at the time. Granted, at that time that was a sufficient answer.
Fast forward a few years later and I found myself on the other end of the joke; asking a new guy to go get me a 10MP fitting to take a measurement. But that brought the question back up -- why not, though? I set out to get answers. This is what I have learned on the subject: In the nineteenth century, many different types of screw threads were required for hydraulic and pneumatic circuits as well as fastening components. As a result, manufacturers started to devise their own fastening systems. This resulted in compatibility problems. The English mechanical engineer and inventor, Sir Joseph Whitworth devised a uniform threading system in 1841 to address the incompatibility problem. The Whitworth thread form is based on a 55-degree thread angle with rounded roots and crests.
This form was selected as a connecting thread for pipes, which was made self-sealing by cutting at least one of the threads on a taper. This became known as the British Standard Pipe thread (BSP Taper or BSP Parallel thread). William Sellers developed the predecessor to NPT; the USS (United States Standard) thread in 1864. This primarily dealt with fastener thread, but pipe thread was subsequently standardized as a result. Sizes of pipe thread increases by the eighth up to ½”, and then by the quarter up to 1½”. From there the sizes increase by ½” all the way to 5” where it jumps to increasing by an inch.
The key take-away here is that pipe thread has been standardized in the US of A for a long time, longer than any other domestic thread standardization as a matter of fact. AN or JIC (largely the same) came about in 1939 when the military decided they needed a fitting to seat metal on metal and not by thread as this would stop any type of leaking that can occur with the standard tapered thread.
At the time, 5/8 thread just was not common enough to be standardized. As machining of pipe and pipe fittings improved, so too improved the machines the pipe was applied to. This resulted in a more diverse range of needs in pipe size and thread types. By the time World War 2 hit, machining had improved enough that 5/8 was common enough a size that the military decided to include it in their the AN system. So, when someone asks you, why is not there such a thing as a 10MP, instead of saying “It just doesn’t. They never made it. I don’t know.” You can confidently tell them. “It just doesn’t. They never made it.”
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