I am plumbing a couple fixtures (a water heater and a shower) with pex and a few threaded fittings (at the heater and shower fixtures only). At first I used teflon tape (3-5 turns in direction of thread), but all the threaded joints were leaking. So I removed all the tape and replaced it with some T2 thread sealant I had laying around. Again, all the joints leaked and some worse than before.

I've also played with different levels of tightness to no luck. Am I missing an essential step in my joins?

What other solutions exist to seal threaded iron pipe or brass joins?

  • You're using iron pipes to plumb a shower to a water heater? That strikes me as a bit odd ...
    – brhans
    May 20, 2019 at 20:18
  • Good clarification, thanks. Just the fittings coming out of the water heater are iron pipe so I'm adapting that to 1/2 inch pex. The shower manifold is actually brass, but threaded so I also have adapters from pex there.
    – Joe B
    May 20, 2019 at 20:25
  • They are both tapered fittings?
    – Solar Mike
    May 20, 2019 at 20:45
  • @SolarMike The water heater has NPT fittings. the manifold IPS.
    – Joe B
    May 20, 2019 at 20:55
  • Just asking - fixed several "leaks" due to some over-eager DIY nut not noticing...
    – Solar Mike
    May 20, 2019 at 20:57

5 Answers 5


Try a hint of pipe dope in the thread valleys with 3-5 layers of PTFE and tighten in stages over the course of 30+ minutes.

Relevant backstory:

I just installed a new water softener and had to use a 1" NPT female to 3/4" pex barb connection between the house plumbing and the resin tank's yoke connection. I tried PTFE tape first and it leaked so I tried pipe dope and it leaked worse. I cleaned it all up and tried PTFE again (though there was some pipe dope residue which might've helped). I went a turn past hand tight and it leaked so I tightened it up. I repeated this 5 or 6 times until I tightened it just enough to stop the leaks. Don't go too tight, because in my experience if you go too tight you can leak. There seems to be a sweet spot. Three days later it's still not leaking.

  • 1
    If you could narrow the focus of your answer by eliminating a lot of the backstory, that would make it much more useful. There's just a lot of reading that was starting to look like you were going to ask a new question instead of providing an answer.
    – FreeMan
    Nov 16, 2020 at 17:43

Never put pipe dope on the threads of the female fitting. It will allow pipe dope which is not water soluble to clog aerators and put contaminants into the water. If the threads on both male and female fittings are threaded correctly and with sharp dies the threads should hand fit up to 2-1/2 threads. Pipe dope and tape can be applied in excess. 4 wraps of tape on the male threads with a quality Teflon tape is sufficient, wrapped over itself clockwise looking from the narrow end of the threads. Or paste pipe dope should be applied to the first 4-5 threads of the male threads and not gooped on. No pipe dope whether tape or paste should get inside of pipe. Not a good thing. Tighten the joint until it is snug, not until it can not be tightened any further. It is a feel or touch thing you will acquire with practice. Turn on pressure whether liquid or gas and test for leaks. If there is a leak and you didn't tighten the joint so tight you hurt yourself you can tighten it little more to stop the leak. If you tightened it so tight there is no more tightening you must untighten the joint completely and start over. PITA. Have been doing this for 48 years and has never failed if threads are good quality threads. Problem today 50% of threads are crap. Remember the fit up. If too loose or too tight get rid of bad fitting.


Thread sealant often has a shelf life of only a few years. Buy new thread sealant.

Wrap a few rows of teflon tape on the male threads, and then apply pipe thread sealant on top of the teflon, then also put some pipe thread sealant into the female threads and wipe the excess away, then install and tighten fittings.

This was the only way I was able to get 1" NPT fittings to stop leaking on a water softener install. Teflon or pipe thread sealant alone did not work.

  • 1
    I'll try this and let you know how it goes. The fittings are lead-free and I just read that they are sometimes less precisely machined than regular fittings. The same post mentioned that using both solved his leaks.
    – Joe B
    May 20, 2019 at 22:06

After trying the above and reapplying, I came across a forum post recommending some blue pipe dope and that worked really well.

Here's a link to the one I used: https://www.amazon.com/Rectorseal-31631-Tru-Blu-Pipe-Thread-Sealant/dp/B0012YO0Z4

I don't know what the difference is between the two, but I'll be using this one from now on.


Are you using inserts into the end of the PEX pipe before putting on the compression fittings? I presume that's what you mean by 'threaded' - they have an olive between the two parts?


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.