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I'm an electrical engineer, working on repairing a somewhat complicated pneumatic system, but I figured this would probably be the best place to ask. This is probably a very basic question to anyone who works in plumbing, but I couldn't find anything about it online.

When you have right-angle pipe fittings, the kind with NPT threads that you screw together, how do you control what angle they screw in to? If you just screw two things together, the relative angle between them will be essentially random as it depends on the exact angle of where the thread starts and how far the threads engage. But we need to put in pipe at specific angles to align fittings to other parts of the system, so how does one actually do that?

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    I had this question myself - I think the crux of your question is, how tight does a fitting need to be, and can I still adjust the angle at that tightness? Maybe this will help- diy.stackexchange.com/questions/12670/…
    – Aww_Geez
    Mar 31, 2021 at 13:56
  • I don't understand the question. Tapered threads (any steel threads, really) self-align as you begin assembly. You'll find that there's a lot of rigidity in the components once they're even loosely assembled.
    – isherwood
    Mar 31, 2021 at 13:59
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    @isherwood If that's the case, then there is no way to control the angle a right-angle fitting will be pointing when it bottoms out. I think the OP assumes the fitting needs to be bottomed out, and would like clarity on that.
    – Aww_Geez
    Mar 31, 2021 at 14:01
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    @isherwood I'm talking about rotational angle. Imagine using two 90° fittings to offset a pipe by a little bit--how do you ensure that the pipes on both ends are parallel to each other?
    – Hearth
    Mar 31, 2021 at 14:01
  • Consider making an edit to add a picture of the area in question.
    – Ecnerwal
    Mar 31, 2021 at 20:31

2 Answers 2

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There is a range between "tight enough" and "too tight" - most alignment for NPT threads is done by using that.

Then again, there are unions and compression fittings (Swagelok, and lesser versions) that remove the connection between the threads and the sealing surface. The vast majority of "complicated lab gas manifolds" I did many moons ago were done almost entirely with Swageloks.

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    i.e. The fitting has to be tight enough to not leak. Once you hit that point, you should have nearly a full 360° of rotation left before you hit "won't turn any further". Therefore, once sealed, you keep rotating until it's in the desired direction and all is good.
    – FreeMan
    Mar 31, 2021 at 14:16
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    @Hearth With some practice one learns the feeling of "it's almost as tight as it'll go." When I get to that point I make a judgement call: if I can get the fitting clocked at the angle I want with up to a half-turn of additional tightening I'll continue tightening. Otherwise I'll loosen it up to half a turn. One can also loosen or tighten the fitting at the far end of the pipe, rather than the present fitting, to make up some of the desired clocking/angle. You might be surprised to find that a joint holds pressure well before reaching impossible-to-turn tightness.
    – Greg Hill
    Mar 31, 2021 at 14:59
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    @d.george I'm afraid you have my line of work wrong--i'm an electrical engineer, not an electrician! I do circuit boards, not conduit. That makes sense though. Unfortunately, I don't think that's an option here.
    – Hearth
    Mar 31, 2021 at 15:26
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    @FreeMan I would have thought backing off would ruin the seal, honestly. Since the teflon has already been compressed into shape, and it's not elastic enough to return to fully seal the thread?
    – Hearth
    Mar 31, 2021 at 15:27
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    I prefer teflon dope to teflon tape these days. I prefered teflon tape to old-school pipe dope in earlier days. You either have to get someone who knows how it feels when pipe is tight enough, or become someone who knows how it feels, and there's always the possibility that you pressure it up, apply soapy water, pressure it down, and take another turn. How's the copper tube connected? That may be another degree of freedom. But assuming this is a "work" project, far more cost effective for work to hire a plumber/pipefitter to help the EE than to pay an EE to become a plumber....
    – Ecnerwal
    Mar 31, 2021 at 15:38
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An extra wrap or two of PTFE tape around the pipe threads can help the fitting to line up snugly in the direction you want.

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