Response to Thread Standards Concerns

| 7/10/2018 10:06:00 AM

Regular contributor Andrew Mackey responds to John Burgoyne (53/3/1: Thread standards) in the April/May 2018 Gas Engine Magazine. Andrew writes:


The studs and nuts on the Oil city-south Penn half-breed engine restored by Andrew Mackey. As with much old equipment, they were non-standard.

Photo by Andrew Mackey

I too have run into problems with inconsistencies in threading. When I restored my Charter-Mietz, the threads on the fuel pump, injector, as well as other connections were tapered pipe fittings. Although the OD of the fittings was the same as NPT US pipe, the threads were definitely not the same. They had the same threads per inch, but the taper was very different. The fittings seemed to be 1/4 inch. “Normal” 1/4-inch pipe in NPT has a thread length of approximately 1/2 inch to 3/8 inch. The fittings on the C/M engine were about 3/4 inch to 7/8 inch long, and the taper on the threads made their angle much less than that of NPT. Although the pipe OD was the same, neither the receivers on the engine nor the NPT fittings would interchange. A machinist friend cut new custom threads onto my NPT fittings.

Another time, I was on a plumbing job in a building destroyed in an explosion. We had to replace nearly 2 miles of 316 1-inch NPT stainless steel process piping that was damaged. We got the pipe and fittings from all over the country. The fittings (thousands of them) were shipped by the crateful. There were two crates that we had problems with. One had 90-degree elbows made in Malaysia. Our threaded NPT piping would not seat in the fittings. The pipe would thread into the fittings to the point where they bottomed out at the back of the elbow and would not make up at all. At first, we thought we had a bad set of dies or that the adjustable die was misset, but we were wrong. We then tried some factory-made NPT nipples, and they did the same thing. The nipples too, buried themselves completely, not making up at all. The socket threading in the elbows was cut too deep.


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