Response to Thread Standards Concerns

Reader Contribution by Staff

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.

Another crate had fittings made in Taiwan. I do not know what their thread source was, but it was totally different from NPT. The thread count was similar to 3/8-inch NPT, and the internal diameter was smaller. You could not even start the NPT pipe into the fittings. We ended up tossing all the “bad” fittings on the scrap pile. The supply houses didn’t want them back, as they were unusable.

Another time, I ran into issues with the restoration of my engine club’s Oil City-South Penn half breed oil field engine. The main bearing mounting studs only fit in the holes where they had been removed from, and the nuts only fit the studs they also came from. I detailed this when I wrote the article for GEM, but didn’t realize there was another issue, as well. Two years later, we had to remove the cross head guides to replace some collapsed paper shims. We re-used the original shims during the original restoration, and now they had to be replaced. During the original disassembly, the studs and nuts were thrown into a box, then reinstalled. This time, when we went to replace the studs – you guessed it, they wouldn’t go. I guess we got lucky the first time. It took over half an hour, trying different holes and nuts on the studs, to get the studs and nuts back in their proper relationship. What a pain!

As the original steam engine had been built about 1880 or so, it probably would not have had standardization of threading as an engine would have today. The cylinder and head mounting were standardized, having been made about 25 years after the original steam engine. By then standardization was becoming the norm, so there were no issues with their reassembly.

Here is some good advice for the readers of GEM: When you disassemble an engine or piece of machinery, keep the studs, nuts and bolts together as a set. Mark and/or note where each stud or bolt came from during the process of removal, and make a drawing so they can be reinstalled in the same hole they came from. Don’t make our mistake and just throw them into a box. Make a note of which hole a stud or bolt came from, and you will save yourself a lot of hassle in the rebuilding process.

Andrew K. Mackey

(862) 432-1552

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