Can a female union tailpiece be used for male NPSM threads?

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Brandon Weiss

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Santa Cruz, California
I have a device that has 1" male NPSM (straight) threads on both ends and I want to adapt it to 1" NPT. The instructions say to get two water meter fittings to make the connection to NPT, but water meter fittings are typically asymmetric (1-1/4" NSPM to 1" NPT or 1" NPSM to 3/4" NPT).

I’m wondering would the female half of a union (like this one) work? Naively it seems like it would, but I don’t know if unions are always (or even typically) NPSM threaded, or if they’re just some random straight threads that aren’t standardized.

Thanks for your help!


In the Trades
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Berkeley, CA
The part that you linked to isn't going to help you. The female threads will be NPT, and the other side will be a flat plate with a o-ring meant to mate with a similar plate on the Watts PRVs listed.

You need a fitting with 1" NPSF threads that accepts a washer for sealing. Since the straight threads won't provide a seal.

While inelegant, if you are forced to use a 1" NPSM to 3/4" NPT part but want to stick with 1" piping, a short section of 3/4" typically isn't going to cause any problems. It will be equivalent to something like 5 times the length of 1" piping.

Cheers, Wayne
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