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HVAC_NW 10-20-2010 05:13 AM

Can't stop leak at 3/4" NPT steel male to brass female
Water pressure: 50 psi
I have steel male thread on expansion tank and a female brass thread on female NPT to female push in adapter.

Not a drop leaks at push in PEX pipe. First, I made the connection using pipe compound that's supposedly good up to 1,500 psi. It leaked like crazy. I tightened it down more. It leaked less, but still a lot.

I redid the connection using 3 turns of Teflon tape and leak was reduced to couple drops an hour, but it still leaks. I tightened using a pair of 12" wrenches probably close to 60 ft-lbs judging by how much force I used.

The length that steel thread screws into doesn't make much sense. It is far from bottoming out the female thread on the brass end and there is plenty of thread left on male thread. What is it stopping on?

Most importantly, why is it still leaking and what am I not doing right?

fabrk8r 10-20-2010 07:53 AM


Originally Posted by HVAC_NW (Post 519962)
I tightened using a pair of 12" wrenches probably close to 60 ft-lbs judging by how much force I used.

Most importantly, why is it still leaking and what am I not doing right?

You answered your own question. You shouldn't need that much force to tighten tapered threads, especially if using pipe dope and teflon tape.

I'd be willing to bet that the brass fitting now has a hairline crack. It may be due to bad threads on the black nipple or possibly cross threading.

HVAC_NW 10-20-2010 07:56 AM

I didn't start off brute forcing. The thread went in smoothly. I didn't tighten that much. It leaked. I took off the shark bite. Tightened the NPT, put back, repeated that another time.

fabrk8r 10-20-2010 08:02 AM

Probably cheap Chinese brass. I've had bad ones before. Inspect it closely with a magnifying glass and when you find the crack take it back and exchange it for a new one. When you have it off look closely at the threads on the nipple to see if there are any bad spots.

Thurman 10-20-2010 11:08 AM

One thing I have noticed over the years is that mass produced male pipe threads on small fittings tend to be "rough" cut. If you were to look at them with a magnifying glass they would be very "rough" textured. Most brass components female threads are cut with threading taps, which are much sharper and more precise than thread cutting dies for male threads. A lot of times it's very hard to get these to seal based on them being "tapered pipe threads" and requires different sealants. An "Oatey" brand comes to mind that worked really will with these. More of a paste than a thin material like most white pipe compounds, and you should let it set for some time before introducing pressure. David

HVAC_NW 10-20-2010 04:31 PM

The water tank is a Watts brand DET-5, brass fitting is SharkBite brand U088 3/4"X3/4" FNPT

But SERIOUSLY, thank god for free rotating SharkBite and PEX pipes for letting me rotate, then remove the tank over and over so easily so I can readjust stuff.

I might just connect a stainless steel 12" water heater braided heater hose (which has shallow depth, then seal is established by the end face with a rubber gasket and immune to thread problems.

The pipe compound that failed MISERABLY (rated 10,000 psi liquid....) at 50 psi is the Oatey Great White

It's non-hardening type and does not specify curing requirements. I applied on male thread liberally, then wiped off from one turn of thread at the top to keep it from getting into system. hand-tightened and it went in smoothly, so cross thread is not likely.

I tightened a little more. It still leaked like a spring.

When I remove it, I can see how far it went from the section of thread where compound is stripped off and it doesn't make sense where it is bottoming out at.

I wrapped three turns of general purpose white teflon tape all the way around the thread and this actually worked better than pipe dope, but still can't get to zero leak.

My skills? Poor materials?

Daniel Holzman 10-20-2010 05:00 PM

Over the years I have done a lot of plumbing, mostly out of necessity. I have had several instances of serious trouble getting brass fittings over steel or iron to seal perfectly. I don't think this is the fault of the pipe dope, remember it is designed to fill in relatively small imperfections, it does not seal damaged threads, gouges, large gaps etc.

As noted, steel threads can be very rough, and may mate poorly with brass. One option is to use three or four wraps of heavy teflon tape (there are three grades of teflon, the white I believe is the lightest, pink is the heaviest as I recall). Often you can only get the lightest grade at the big box store, you may need a realy plumbing shop for heavier teflon, which may work a lot better in your case.

Although research I have read suggests that the combination of teflon and pipe dope is no more effective than teflon tape alone, I sometimes use the combination anyway, with the pipe dope over the teflon tape. I don't really know if it helps any, but I recently installed a water expansion tank using copper fittings into a steel tank that had a plastic female fitting, and I needed multiple tape wraps to get a good seal. Because it was a plastic fitting, I use no pipe dope.

HVAC_NW 10-21-2010 01:21 PM

I exchanged the expansion tank for an identical one. I replaced the Shark bite brand fitting with a different brand one. I used three turns of pink full density PTFE tape. It appears leak is corrected at static pressure at incoming temperature, as well as under higher pressure at higher temperature (simulated worst case condition with incoming shut-off turned off, then water temperature setting raised to allow expansion tank to work)

secutanudu 10-22-2010 02:17 PM

I use "Blue Monster" teflon tape - it's awesome. I find that with the cheap white kind, i need like 5-6 wraps to prevent leaks.
The blue stuff is thicker and not as stringy when you tear it. So easy to work with.

WDR 10-28-2010 10:45 PM

I like the blue monster tape as well, unfortunately I have not had a chance to get some from the plumbing supply store recently. As for pipe dope I was told rector seal 5 was good. I have had used it and have good luck with it, it works a lot better if you let it sit a couple days to harden before applying pressure.

Homerepairguy 10-28-2010 11:38 PM


Originally Posted by HVAC_NW (Post 519962)
Most importantly, why is it still leaking and what am I not doing right?

What works for me when I have similar problems is using both pipe dope and teflon. I coat the male pipe threads with dope, wrap it with teflon going from the base of the threads to the tip and back to the base, coat the teflon with dope, attach.

Be sure to wrap the teflon tape clockwise when looking down at the tip of the pipe so it won't unravel when tightening the pipe.

Maybe it'll work for you,

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