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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 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 at 13:59
  • @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 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 at 14:01
  • Consider making an edit to add a picture of the area in question. – Ecnerwal Mar 31 at 20:31
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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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  • This, I'm afraid, is not a lab gas manifold, but a piece of specialized equipment that needs a valve replaced. There's not room for anything more than exactly what was in there before, which was a right angle fitting, a short length of copper tube, another right angle fitting, and a quick-disconnect that plugs into a desiccant column. No room for anything other than that. – Hearth Mar 31 at 14:03
  • The first paragraph covers that. Thread compound or PTFE tape create a cushion of sorts that allows at least a full rotation of adjustability. Watch any video on black iron pipe assembly for clarification. – isherwood Mar 31 at 14:14
  • 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 at 14:16
  • @FreeMan How can you determine when it's tight enough to not leak? I can't attach it while under pressure, because it can't be under pressure until it's fully assembled; the pressure path just doesn't give any way to connect it to pressure without also fully assembling it. – Hearth Mar 31 at 14:43
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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 at 14:59

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