I am trying to connect a spigot (3/4 in. FIP Inlet) to an elbow (3/4" NPT Male).

If I understand correctly, FIP and NPT are directly compatible - right?

Even with paste thread sealant (which I believe you are supposed to use for this kind of connection), and tightening it wrench tight, it still leaks. Not spraying, or even dripping, but just pooling up water enough to drip one drip every several seconds. This seems wrong. I noticed that when I unscrew the threads, they are really clean (i.e. the paste sealant seems to have been pushed out entirely when I connected the two).

Am I doing something wrong here?

  • They are tapered threads. May be the parts did not come to the end so the gap left or bad quality.
    – user263983
    Apr 25, 2021 at 20:43
  • 1
    It's either your technique/materials not being up to snuff, or poorly made fittings (some of the junk they sell these days is extremely poorly made/defective right out of the package.)
    – Ecnerwal
    Apr 25, 2021 at 22:23
  • If it is dripping outside onto the ground and not into the wall, then let it drip for up to a week. Sometimes "seeping" joints seal up as particles of joint compound are pulled into place. I jhave had to use both Teflon tape and joint compound to get a seal. Normally you put joint paste only on the male threads, but on one outside faucet I had to apply joint paste to both the male and female threads. All else fails get some plumber's hemp fiber and wrap it on the male threads before applying joint paste to the male threads. Apr 25, 2021 at 23:24

2 Answers 2


I put H B Smith 350, 450, and 650 screw nipple boilers together with RTV for many years as did a couple contractors I worked with. These boilers almost always had nipple thread leaks. When I or the contractors used RTV we never had a leaking nipple.

my 2 cents


Try this; use an RTV silicone on the threads instead of joint compound or teflon tape. Put it together and wait a couple days to turn the water back on. That should work and stop the leak but if it doesn't work get some Expando, a gray powder that you mix with water, mix it to a thick paste and apply as the pipe dope. When it dries, it expands and will seal just about anything. Just a note, I have had better success with the RTV than with Expando.

  • So RTV silicone works better than something like Rectorseal Tplus2? From what I can tell it seems like Tplus2 is specifically a thread sealant, while RTV silicone is more "general purpose" (also for sealing windows, etc.). Typically I'd imagine something made for a specific application would be better, but hey, I'm the one that's having the problem so listening to me is probably not advisable ;) Apr 26, 2021 at 12:57

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