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I've heard that to join threaded gas piping in my house, I should use either pipe dope or teflon tape rated for gas. Regarding the dope, I've been told to dope "all but the last two threads." My confusion, though, is which two are considered the last two?

Do I not put dope on the two threads

  1. closest to the pipe edge or
  2. closest to the pipe center?

i.e., is "last" referring to the threads closer to the edge or center of the pipe?

enter image description here

FreeMan
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theforestecologist
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  • Also, why? Why not dope the last 2 threads? – theforestecologist Jul 17 '23 at 03:23
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    More to the point - if you need to ask, you're not qualified, and should leave this work to someone who is. In fact, in most parts of the world, it's illegal to mess with gas fittings! – Tim Jul 17 '23 at 14:55
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    @Tim that's foolish. It's just fitting two pipes together. I intuited the given answer and rationale without issue. I'm just cautious and smart enough to solicit community wisdom as i venture into new projects. I appreciate the helpful responses. It's always good to hear others perspectives (often times quite variable btw). Besides, it doesn't take much to discover the limitations of scope/awareness/expertise of contractors in my area, so they are no more qualified than I am other than the fact that they've done a bad job many more times than I have! – theforestecologist Jul 17 '23 at 16:41
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    It may well be foolish. I have done plenty of gas fitting with no problems. That doesn't negate the problems I might have had if something had gone wrong, and the insurance decided it was my fault the house exploded. Not saying you can't do it, instead, saying the rules are there for a purpose, to save people from themselves and.. their inexpertise. Go ahead by all means, but be aware that if there is a problem, the law won't be on your side, believe me. – Tim Jul 17 '23 at 17:14
  • @Tim, true, but it's not like the law is really ever on my side (i.e., the side of the "little guy") to begin with! :p. Unfortunately, it's all about who you know and how much you pay them -_- – theforestecologist Jul 17 '23 at 18:36
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    The small advantage of having a little dope/tape on the area marked #2 helps show a future person how the pipes were sealed, because it is visible outside the joint. – Criggie Jul 17 '23 at 22:54
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    @theforestecologist The "just fitting two pipes together" attitude is what concerns me. There's also pressure testing and leak checking and a whole host of other things which go into "fitting to pipes together in a way which won't result in a giant fireball" -- The concern is that it's what you don't know will get you in the end. This is something that hopefully would have been included when you learned how to do it properly. The fact that it wasn't indicates that you were missing something in the training. So you need to ask yourself -- What else aren't you aware of? – R.M. Jul 18 '23 at 01:50
  • @R.M. I have no training. That's why I info gather out the wazoo. I mean I have a bunch of water pipe plumbing experience (offered a job by a plumber for my quality pipe sweating), but I've only assisted a friend in doing gas once. I've leaked check with dawn soap multiple times, though,and certainly know to do that in this situation. My question for you is what else should I be aware of that you don't think I'll pick up through researching the process? (btw, I'm just hooking up a hot water tank for now) – theforestecologist Jul 18 '23 at 04:01

1 Answers1

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It's been awhile since I worked with gas pipe, but my understanding was that your area marked as (1) are the last 2 threads.

The reason is so there is no ribbon or tape hanging off the threads that can break off and go upstream and clog something.

What I prefer for sealing any gas pipe is Rector Seal

RMDman
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    The reason for not doing those two threads is right on. I kind of have a problem with calling them "last". My sense(common or just weird) are they are the first threads. The first that are made by the die, the first that go in a nut or fitting, the first at the end of the pipe. – crip659 Jul 17 '23 at 10:35
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    The threads marked 2 will never go into a fitting, due to the nature of tapered pipe threads (and are the "last" threads.) See https://diy.stackexchange.com/a/53694/18078 regarding threads showing on a properly tightened fitting. – Ecnerwal Jul 17 '23 at 12:52
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    So there might be good reasons to avoid doping the last 2 threads in both ends. – jpa Jul 17 '23 at 13:03
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    @RMDman The link to Lowes is already dead (at least for me). Do you mean Rector Seal Blue Plus? (That is the only one that specifically mentions natural gas in the description blurb, unlike #5) https://rectorseal.com/rectorseal-blue-plus-group/ – Seth Robertson Jul 17 '23 at 14:52
  • @Seth Robertson, The link still works for me. I'm far from techy in computer "stuff", so don't know what to do there. I use #5 because it is in my toolbox and it is suitable for a number of liquids and gasses. if you prefer the Blue, go for it. – RMDman Jul 17 '23 at 15:54
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    That link gives me: You don't have permission to access "http://www.lowes.com/pd/RectorSeal-No-5-Pipe-Thread-Sealant-4-fl-oz-Sealant/5002633545" on this server. – user20637 Jul 17 '23 at 19:11
  • The lowes.com website itself works but none of the products show up. Probably a geographical block, are you from Europe by any chance? I am... Anyway, just google for RectorSeal No. 5, there will be plenty of other hits -- including the manufacturer's own site. – Gábor Jul 17 '23 at 20:32
  • Changed the link to one from another company. But if anyone googles Rector Seal it will come up unless your country doesn't allow it. – RMDman Jul 17 '23 at 23:45