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Are two different pipe unions in a given trade size (1/2, 3/4, etc.) generally compatible with each other?

e.g. if you get a new 'half union' would its physical characteristics generally mate up with a pre-existing opposite-side half union from another manufacturer?

Note - I'm not asking about how the union mates with the pipe itself, be it threaded, sweat, etc. Just how the union halves attach to each other.

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As far as unions from different mfg's of different sizes being compatable my experience is no they are not, even the same brand I checked going from 3/4 to 1" would not mate up I was able to find one that was 3/4 on 1 side and 1" on the other but next time I will probably just use a reducing fitting as the "special" union was more expensive than a standard 1" union and a nipple + a reducer but I had a space limited amount of room to make the repair so in that case I paid the extra. Also make sure if going from copper to galvanized to use a dielectric union. The failure of a home owner to do this in the past has caused plumbing failures in under a year in past repair jobs.

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As far as the two halves of a union joining together there is a smooth mating surface which must be a near perfect fit. There is some room for deflection. In my experience this mating surface is usually different with each manufacturer of pipe fittings. If you have half a union from one manufacturer the threads may or may not match up and form a water tight seal with the other half of the union from another foundry.

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Union's are not compatible. Unlike the pipe side, the mated side of a union is not standardized. Even if the threads are the same (as there are only so many available) the mating surface may not be. You could have a leak free connection that seems tight but can't withstand lateral pressure.

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They should. We get unions manufactured in other countries, different companies and they all work the same. Some might be larger, say 300lb ones compared to 150lb ones, but the holes will still be the same. We grab the first one in the drawyer. It could be 20 years old or 2 years old.

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  • As a general rule, North America uses different standards to the rest of the world - applies to things from paper sizes to pipe-threads (NPT in north america, ISO elsewhere).
    – Niall
    Jan 29 '19 at 7:33
  • British Standard Pipe (BSP) is common in much of the world, even if it's now officially not that in some places. Jan 29 '19 at 11:59
  • @SomeoneSomewhere Yeah, the BSP whitworth standard was adopted to form the ISO standard for pipes and fittings.
    – Niall
    Jan 29 '19 at 19:32
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In the US, the ANSI National Pipe Thread standards specify thread pitch, diameter, shape, etc. to ensure compatibility. In other parts of the world, ISO standards apply.

Trade size alone does NOT determine whether threads are compatible; for example, tapered threads may not make a reliable seal well with straight threads. There are lots of different thread standards, although only a few are seen in common residential applications.

The current Wikipedia page for National Pipe Thread has detailed information.

The question has been edited to ask whether the halves of a union are standard and compatible. Short answer: pretty sure they are not.

There are three thread matings on a threaded union - the threading on either side that accepts the pipe's threads, and the internal threaded mating between the two halves of the union.

I suspect there is no standard for the internal construction of a union fitting. The seal between the two halves is a tapered seat. I believe straight threads are used between the two halves of the union so the mating at the tapered seat is tight.

In any event, the angle of taper of that seat and the pitch and position of the threads, and possibly the type of thread, could prevent a good seal between halves of different brands.

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  • Do unions use tapered threads though? Jan 29 '19 at 12:27
  • @daveincaz - a union made for plumbing has three thread matings - the two female NPT that accept the pipes, and the internal threaded mating between the two halves of the union, which might be tapered or straight - I don't know - but not a compatibility issue. editing my answer... Jan 29 '19 at 12:41
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    I'm referring to the mating surfaces of the unions themselves, not how they attach to the pipe. That's what the question is asking (I did make an recent edit to emphasize that). Jan 29 '19 at 12:42

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