Key Takeaways

• Pipe thread does not come in a -10 (5/8″) size because the size was never standardized for National Pipe Thread (NPT)

• Pipe thread sizes were standardized in the 1860s, increasing by eighths up to 1/2″, then by quarters up to 1-1/2″, then by halves to 5″, then by inches

• The 5/8″ size was not common enough at the time of standardization to be included

• The military’s Army-Navy (AN) / Joint Industrial Council (JIC) system, adopted in 1939, did include a -10 (5/8″) size because machining had advanced enough by then

The Joke Every Fitting Technician Knows

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.

How Pipe Thread Was Standardized

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.

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 (BSP) thread (BSP Taper or BSP Parallel thread). William Sellers developed the predecessor to National Pipe Thread (NPT): the United States Standard (USS) thread in 1864. This primarily dealt with fastener thread, but pipe thread was subsequently standardized as a result. Sizes of pipe thread increase by the eighth up to 1/2″, and then by the quarter up to 1-1/2″. From there the sizes increase by 1/2″ all the way to 5″ where it jumps to increasing by an inch.

The key takeaway here is that pipe thread has been standardized in the United States for a long time, longer than any other domestic thread standardization. Army-Navy (AN) or Joint Industrial Council (JIC) fittings (largely the same system) 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.

Why 5/8″ Was Left Out

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 the AN system.

So, when someone asks you why there is no such 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.”

Need Help?

Contact your Royal Brass and Hose local sales representative or call 800-669-9650 to speak with a member of the customer service team about your thread identification needs.

We have 11 locations across the Southeast and Midwest, including Knoxville, TN; Marietta, GA; Orlando, FL; Charlotte, NC; Little Rock, AR; Fort Worth, TX; Benton, IL; Madisonville, KY; Princeton, WV; Birmingham, AL; and Calvert City, KY.