# Resealing of black iron union connection



## hvac benny (Dec 29, 2009)

All you have to do is re-tighten it. Be sure to test it for leaks afterwards.


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## Alan (Apr 12, 2006)

I like to put a very thin skim coat of pipe dope on the mating surfaces even on new unions. Sometimes they aren't perfect, and when they're old, they're even less perfect.


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## hvac benny (Dec 29, 2009)

Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary. 




Alan said:


> I like to put a very thin skim coat of pipe dope on the mating surfaces even on new unions. Sometimes they aren't perfect, and when they're old, they're even less perfect.


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## Alan (Apr 12, 2006)

hvac benny said:


> Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary.


I will keep that in mind for my own personal judgement. But if my boss tells me to put dope on the mating surface, I will. :thumbup:


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## Ishmael (Apr 24, 2007)

Alan said:


> I will keep that in mind for my own personal judgement. But if my boss tells me to put dope on the mating surface, I will. :thumbup:


I've done it the same way you do for the last 15 years. The master I apprenticed under has been doing it for about 25 years, and he learned it from a guy who did it for 40 years and so on, and so on...

I'm trying to figure out why pipe dope is OK on the threads, but not on the union. :wink: The only reason to put pipe dope on the threads is to fill in the imperfections in the threads - the same rationale for using it on the mating faces of a union.


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## hvac benny (Dec 29, 2009)

Ishmael said:


> I'm trying to figure out why pipe dope is OK on the threads, but not on the union. :wink: The only reason to put pipe dope on the threads is to fill in the imperfections in the threads - the same rationale for using it on the mating faces of a union.


It's because the threads of a union aren't what create the seal.


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## TheEplumber (Jul 20, 2010)

hvac benny said:


> It's because the threads of a union aren't what create the seal.


But what creates a seal on a coupling?
I put pipe dope on all my unions too. As well as compression ferrules and nuts, ground joints on p traps. A little dab is all. :sneaky2:


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## Alan (Apr 12, 2006)

hvac benny said:


> It's because the threads of a union aren't what create the seal.


He's talking about : Why is it ok to put pipe dope on a threaded connection, but not on the mating surface of a union.


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## Ishmael (Apr 24, 2007)

hvac benny said:


> It's because the threads of a union aren't what create the seal.


We were talking about putting pipe dope on the mating surfaces of a union. And you said, "_Pipe dope on unions, and any other similar fittings, is a HUGE no-no_". Why?


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## hvac benny (Dec 29, 2009)

Re: Ishmael's response: Pipe dope is a no-no because it dries out and/or hardens over time. Unions are a purely metal on metal seal, just like a flare fitting, and as such do not require dope. If they do, something is wrong. Do any of you use pipe dope on flare fittings? I sure hope not, although I have seen dope and even tape on a flare fitting, which I red tagged. The only substance that should be used on these sorts of fittings is oil or vaseline, which doesn't dry or harden, and enables the union to be removed later on as it will prevent seizing. You guys can do whatever you want with your own fittings, but I posted here to let DIYers know that it is not the proper way to do things. The fact that some have been doing it for decades makes no difference, it's wrong.


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## ddawg16 (Aug 15, 2011)

Is this by anychance for gas? If so, I hope this union is not under the house or in a wall. It's my understanding that any union for a gas pipe has to be outside.

As for the union....the mating surface is a machined match and I believe one side is usually a slightly softer material so that it conforms to the other side for a better seal.

Also understand that pipe dope and teflon tape are not what is doing the sealing....they help a little...but their real purpose is to lubricate the threads so that you can get more turns on the pipe and better compression of the thread connection. 

Case in point...see how many turns you can get with a dry thread....then do it with teflon or pipe dope. Also, see how hard it is to break that dry connection loose vs the dope or teflon connection. Same thing with stainless steel connections....you need to use antiseize compound, otherwise, the SS connection will never come loose....and will most likely leak...


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## anesthes (Jan 21, 2011)

hvac benny said:


> Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary.


They are machined in pairs?

I think I found my leak!

I put the female side on, and when I went to install the male side the threads were junk. So I grabbed another male and put it together, without changing it's mate on the other side.

-- Joe


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## COLDIRON (Mar 15, 2009)

They are called Ground joint unions for a reason. A little oil or Vaseline OK. Nothing else.


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## listen&learn (4 mo ago)

Alan said:


> I will keep that in mind for my own personal judgement. But if my boss tells me to put dope on the mating surface, I will. 👍


 It's time to change bosses before you get involved in an accident.


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## listen&learn (4 mo ago)

anesthes said:


> They are machined in pairs?
> 
> I think I found my leak!
> 
> ...


They are not machined in pairs. If they were the cost for a 3/4 union would be $35 and not $6. The steel is soft enough so the mating surfaces form a proper seal. For critical applications they are tested individually at the factory for leaks and also if you look at the manufacturer's specs they specify the torque that should be used on these unions.
As mentioned before, do NOT use dope or anything else other than light oil or vaseline or light grease to seal a union. Unions are for providing easy service access. Using dope they MAY seal the first time but they will NEVER EVER seal again if you loosen them and you will need to replace the union. If you insist using dope then use a coupling instead of a union. After all a 3/4 coupling costs only $0.80 versus a $6.00 union. 
Food for thought.....


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## Ghostmaker (Mar 2, 2013)

hvac benny said:


> Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary.


Never had that problem been a plumber all my life...But then again it was rectum seal .


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## joed (Mar 13, 2005)

12 year old thread. Hope it is fixed by now.


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## anesthes (Jan 21, 2011)

joed said:


> 12 year old thread. Hope it is fixed by now.


Long ago. Was just a bad fitting. 

-- Joe


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