# Black pipe gas unions: install "wet" or "dry"?



## hvaclover (Oct 2, 2008)

They are a true metal to metal seal. 
When you buy a ground joint it comes loosely threaded together. never mix and match male and females even if from the same manufacturer. 
There could be a slight difference int way the threads are cut and they won't tighten.

Concerning pipe dope, use it as a last resort. Sometime even matched union pieces can leak slightly so in this case yes, but it is not considered good craftsmanship.

I have only done it a handful of times when i could not get a new union.


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## hvac benny (Dec 29, 2009)

They're a true metal-to-metal seal so sealant should not be used under any circumstance. When I did HVAC I was taught that sometimes a flare fitting would need a small amount of hvac oil on the male end to allow the flare nut to turn that extra little bit if a leak free fit can't be achieved, even with a perfect flare. At the gas utility, we use petroleum jelly on our unions and meter spuds. The difference between the use of sealant and oil/jelly, is that the latter is a lubricant and allows the mechanical coupling to turn just a little bit more. the petroleum jelly also makes it easier to undo the coupling later on, whereas the sealant fills the threads and prevents the coupling from being fully tightened; once the sealant dries up it will most likely leak.


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