Problem with 3/4 copper unions

[Steve-Maine]

I have been running a 3/4" copper air line in my shop. I can"t seem to get the 3/4" copper unions to stop leaking. They are leaking around threads not solder joints.I bought Nico brand at HD made in USA. Would teflon tape and paste help?? Thanks for any help..
 

[Ken Macfarlane]

The threads on a union don't hold in the pressure, they just draw the mating surfaces together. Don't put paste or tape on them. You can lube the threads to make it easier to tighten.

Open up the union, see if there are burrs on the bell or boot (right terms? the convex and concave mating surfaces). They also can't have misalignment. You need to crank them down tight for air service.

In my old shop I replaced them with straight couplers. No leaks after that, could leave the compressor valve open.
 

[showcrop]

I would have a problem with a union seating once in a great while. I always had silicone grease in my box so I would smear a dab on one side of the mating surface, which enabled it to seat a little better, and almost always stopped the leak.
 

[circus]

"Would teflon tape and paste help??" Yes
 

[Mike (WA)]

You don't really need unions with sweated copper- just "unsolder" a joint if you need to take it apart.
 

[Brian G. NY]

Ken is correct.......I have noticed that the China made plumbing "fittings" just don"t "fit" like the old USA fittings.
 

[glennster]

if you are going to use the airlines for spray painting, i would not use silicone. it will cause fish eyes in the paint.
 

[Steve-Maine]

To Mike,
I should have used connectors in the beginning. Everyone of the unions leaked. I'm now in process of cutting them out. Thanks
Steve
 

[showcrop]

[Dean Olson]

I was taught to use pipe dope on mating surfaces and threads of unions as an apprentice plumber 30 years ago. We tested all systems with air before turning water or gas on. I remember very few leaks and never a problem with unions other than not getting them tight enough. Nico or did you mean Nibco. Nibco is the preferred brand of copper fittings.