Tap Terminology: For Light Fixture Threaded Tube

[MrWhoopee]

Interesting story. I am surprised that the gas would have piped with straight threaded pipe and fixtures. It seems that tapered threads would have been a lot better, a lot easier to effectively seal against leaks. And even the expansion of the abbreviation (NPSM = National Pipe Straight Mechanical) speaks to the thread being used for MECHANICAL purposes, not for pipes carrying a liquid or a gas.

But then, strange things do and did happen.

I'm fairly sure they were NPT (tapered) when used to plumb gas but transitioned to straight when adapting fixtures to electric. It's pretty easy to just keep turning the NPT die to produce a straight thread, I've done it. Not sure it would meet NPSM specs, but it would hold a lamp together.

[nickel-city-fab]

I dunno, I really don't. Audel's mentions that the Briggs standard went all the way back to the 1860's but it also mentions straight threads.
I would guess that it all depends on when local building codes were developed. Roughly at the same time as the ASME.
I was trained for ASME work in welding and fabrication --
did you know they were originally sponsored by the Hartford Boiler Insurance Company?
Back in the days of steam.

So I wonder what came first.... Tapered pipe thread or straight pipe thread.

-D

[MrWhoopee]

Taper threads came first.

A Brief History of Pipe Threads

https://ultimheat.com/s3-museum/2019%20History%20of%20threaded%20pipe%20connections%2020190221.pdf