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Thread: What makes compression fittings leak?

  1. #1
    DIY Senior Member guy48065's Avatar
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    Default What makes compression fittings leak?

    I was pressure testing a repair I made on a faucet and got bubbles. Not from my solder, though. The 1/2MIP to 3/8" compression fitting was leaking at the crimp band AND the taped threads. All fittings are brand new, undamaged and had good clean threads. The ferrule was tightened until there was a visible "waist" on the 3/8 copper tube (new). Am I doing something wrong or:
    1 - should I dope up everything with paste as a mater of practice? or
    2 - will this be leak-free to water and only leaks thinner air?

    First picture shows the setup. 80psi is trapped in supply line and placed underwater.

    Second picture is taken underwater and shows the bubbles forming all over this wrench-tight fitting.



    Romeo and Atlanta, MI

  2. #2
    Jack of all trades DonL's Avatar
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    Probably because the No lead connections are to hard of a material to compress properly.

    And the Metric conversions are just enough off that a good seal is not possible unless you over tighten them.

    Just my guess.

    Tighten just till they break then back them off 1/4 turn, Then buy a good one.


    Looks like your setup is using a flare fitting ?


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    Last edited by DonL; 10-06-2013 at 09:52 AM.
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    Forum Admin, Expert Plumber Terry's Avatar
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    Are you sure that's even a tapered thread on the chrome adapter. May fittings like that for faucets are straight threads that are meant to pull up tight. Not snug up to a taper.
    Lav supplies and lav nuts are cut and threaded straight, without a taper.

    The compression side with sleeve; those I use some light oil and tighten up.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If you use tape on an actual compression fitting's threads, THAT will cause it to leak! Tape or dope is only required on a tapered threaded plumbing fitting. Straight threaded connections rely on either compression or a gasket of some sort to make the seal, and the threads and nut are only to compress things together to create the seal (and thus, are not tapered and require no sealing material in them).
    Jim DeBruycker
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    DIY Senior Member dj2's Avatar
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    If I face a situation like this, I immediately go to plan B: replace the compression with a male adopter.

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    DIY Senior Member guy48065's Avatar
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    The chrome adapter was tagged 1/2"MIP - 3/8" compression. It's not a fitting that came with the faucet--it's part of the pressure test jig. When I'm ready to use the faucet I'll cut the copper tubes shorter and use a pair of braided 3/8 - 3/8 hoses to connect to the stop valves.

    The 1/2" tapered pipe end of the fitting has 2 layers of tape on it. I'm surprised it leaks--brass and plated brass normally have nice smooth threads with no voids. As for the compression band I suppose I could just keep cranking on it & see if it stops but that just shouldn't be necessary--the parts are new, clean and dry. Maybe a dab of oil or grease is a good idea, but I thought the crimp of the brass ferrule on the bare copper should be done dry to insure they fuse together.
    Romeo and Atlanta, MI

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    Jack of all trades DonL's Avatar
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    May be the difference is made in USA and made in China. The pipe bottoms out before the threads are tight.

    If you do like Terry recommends , a little lube will let you get the compression tight without breaking it.


    Good Luck
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    Moderator & Master Plumber hj's Avatar
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    What do you mean by a "waist" on the tubing? Once you deform the copper, it is almost impossible to NOT have a leak until you torque it even further. Chrome fittings may leak because the plating on the threads can cause an incompatibility.
    Licensed residential and commercial plumber

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    DIY Senior Member guy48065's Avatar
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    Yeah I was wondering about the chrome but that was the only type in stock. I never have issues with brass fittings.
    What do you mean by a "waist" on the tubing? Once you deform the copper, it is almost impossible to NOT have a leak until you torque it even further.
    That's the point of my question--even something this simple has a right and a wrong way to do it and I may be doing it wrong. It's not like fittings come with instructions...
    The ferrule HAS to deform the copper or it won't fuse to it and seal without slipping. It can potentially leak between the ferrule and tube, between the ferrule and fitting seat, or between the ferrule and cap. That's a lot of surfaces on one connection. I'm assuming this is a fluke (since there are no "happens all the time" replies) but both supply lines leak at the same rate and since the fitting only comes with 1 ferrule & cap, the 2nd set came out of a bag of replacements that aren't even from the same mfr.

    Do the 3/8"OD fittings on hose (SS braided or reinforced vinyl) seal any better?
    Romeo and Atlanta, MI

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    DIY Senior Member guy48065's Avatar
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    I think I need to hit a different supply and find a solid, unplated, brass adapter.

    Or just drop it--I'm satisfied my solder isn't leaking.
    Romeo and Atlanta, MI

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    Forum Admin, Expert Plumber Terry's Avatar
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    I use a fitting like this. Though I guess that's going to change soon.

  12. #12
    DIY Senior Member guy48065's Avatar
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    ??
    Going all plastic & crimps?
    Brass is now a precious metal?
    My leaky fitting has persuaded you to give up on compression fittings?
    Romeo and Atlanta, MI

  13. #13
    Forum Admin, Expert Plumber Terry's Avatar
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    My leaky fitting has persuaded you to give up on compression fittings?
    We use compression fittings all day. No issues.
    Brass is bein phased out in 2014 because of lead on potable water.
    Once more reason so many faucets are coming with plastic supply lines.

  14. #14
    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Terry View Post
    We use compression fittings all day. No issues.
    Brass is bein phased out in 2014 because of lead on potable water.
    Once more reason so many faucets are coming with plastic supply lines.

    That does suck. I think.

    There is nothing wrong with a compression fitting if installed properly.

    I will take lead over plastic any day. Plastic can make you sick also.


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    Moderator & Master Plumber hj's Avatar
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    quote; The ferrule HAS to deform the copper or it won't fuse to it and seal without slipping.

    Incorrect. The edges of the ferrule press against the tubing to seal. IF they deformed the copper, it would be VERY difficult to "pull' the ferrules and the deformation would prevent new ones from sealing. Neither of which is the case in a proper compression joint. (In fact some will just slide off once the nut is loosened if it is type "L" or "K" tubing.) Tubing is very strong, UNTIL you deform it and then further deformation occurs very easily which is why you would have to tighten it VERY excessively to get it to seal.
    Licensed residential and commercial plumber

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