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Thread: How to judge tightness of threaded connections

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    DIY Junior Member WendyH's Avatar
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    Default How to judge tightness of threaded connections

    My new Moen shower valves have female threaded ports. I am sweating male adapters to my pipes and then connecting to the valves. I then have further sweat connections between valves and to the supply lines and shower heads. I won't be able to really test for water tightness until the system is complete, but by then I won't be able to disassemble the threaded connections. How can I tell if they are correctly tightened? Is it possible to overtighten? The connections are rigid copper to brass, 3/4", no compression. I'm using tape on the threads.

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    Moderator & Master Plumber hj's Avatar
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    It would be almost impossible to overtighten, unless you are a 600# gorilla. There is NEVER any guarantee that the connections will not leak, regardless of WHO tightens the thread. That is why few plumbers use the valves with threaded connections.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Many (not all) valves with threads will also act like a fitting (i.e., the pipe will fit inside the valve) and you can solder it. Because the valve body is bigger, it will take longer to reach soldering temperature and can transfer heat to the guts, so it is often recommended to remove the guts while soldering. This is a more reliable method, if available.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    quote; Many (not all) valves with threads will also act like a fitting (i.e., the pipe will fit inside the valve) and you can solder it

    That is only true for those valves with a mip connection. FIP connections ALWAYS require a threaded fitting attached to them.

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    DIY Junior Member WendyH's Avatar
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    I just got home and checked. The pipe fits into the valve, but it seems to be quite loose (but a coupler is too big to fit). Plus the females are giant thick brass beasts. I imagine they would be hard to heat and require a ton of solder. And then I'd worry about solder running into the valve itself. I'm thinking that the chicken way is the best for this amateur. I'm also thinking that I could probably use our compressor and some plugs to pressurize the pipe and then bubble test the threaded connection. I don't imagine it would take too much pressure.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    For a belt and suspenders thing, use both the dope and the tape. If the tap was getting a little dull, it can tear the threads and make it hard to seal with just one - it won't hurt to use both.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member K.'s Avatar
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    While it's certainly true that pipe threads are not guaranteed to be watertight without the right amount of dope and/or tape, you can at least be certain about how many turns to make. All tapered thread specs have an 'engagement' spec which tells you the overlap between the two parts when fully tightened. You can convert that into a number of turns by using the threads per inch spec.

    -Jonathan

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    Moderator & Master Plumber hj's Avatar
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    ANYONE who depends on a "formula" to determine how tight the joint is, is almost guaranteed to have a leak. Any "engagement" spec will be for "ideal" conditions as determined by some engineer in a perfect, spotless lab, (possibly NEVER water tested to see if it was actually leakproof), and has NOTHING to do with conditions in the real world.

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    DIY Junior Member K.'s Avatar
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    Ah, the engineers are one step ahead of us... there are different thread categories for dirty and clean environments. As far as standard pipe thread goes, I don't know which category is in but since overtightening will not close a leak in a tapered pipe joint, why not be aware of how far it is useful to tighten? Plus some finish and trim fittings have to be threaded on to end up at a particular angle... so do you stop a half turn short of the spec or go a half turn past it? Anyway, just thought I'd throw it out there since it was a big help for me.

    Cheers!

    -Jonathan

  10. #10
    Moderator & Master Plumber hj's Avatar
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    quote;but since overtightening will not close a leak in a tapered pipe joint,

    And WHY would tightening it further NOT stop a leak in a tapered joint? It is TAPERED, so another turn or two WILL create more "tightness". A straight thread, however, would not do it since it needs something besides the threads to seal it. IF the piece can be tightened that additional half turn, more or less, do it. Otherwise, redo the seal and stop before you reach that point.

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