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I have a 3/8 OD (I believe) water supply line coming out of a T-connection in my kitchen - one line services the cold water for the sink faucet and the other goes to the fridge.

I would like to cap the line that goes to the fridge/ice maker (but I don't want to remove the T all together because I want to preserve the option for later).

I went to the hardware store and was directed to the compression cap in the photo but this doesn't feel like the right solution. I would be tremendously appreciative for thoughts on:

-Is this the right solution?

-Should I be worried that it just metal on metal without a washer or anything else to seat the cap?

-Should I use teflon tape?

-Someone mentioned that this item was not to be used solo but rather with a ferrule. Do you agree? How would that work?

-The packaging is unintelligible to me - it says to "fit nut onto tubing and then insert sleeve" and "insert tubing into fitting." I don't know what this is referring to but it leads me to think that this is not designed for the use I have in mind.

I am very nervous about water leaks and want to take the most judicious approach.

TL:DR version: We don't use the ice maker in the fridge and the current water supply line going to the fridge is rusty and not looking good. Rather than replacing the line (I also fear that my freezer might have a small leak in the line but that is for another post) I figure capping it is the best way to avoid possible leaks in the line or a leak in a line once the water enters the fridge. Also, I don't want to remove the T-connection that splits the main cold water line to the sink and the fridge because we may be getting a new fridge and we may want to use the ice maker then... Are there other clever solutions to this problem?

3 Answers 3


The back of the cap packaging is generic and is meant more for other compression fittings. It doesn't make sense for a cap.

That cap is the correct part you need, but Teflon tape is not needed for those threads. In compression fittings, the threads do not seal anything. The seal is made between "compression" of a ferrule that slides around the pipe being connected to.

With a cap, there is no ferrule, and the idea is that the seal will be metal to metal - the flat surface on the male threaded side of the tee, and the flat inside of the cap. That cap will need to be tight to make a good seal. If the face of the male threaded part is damaged or roughed up at all, a good seal might be hard to achieve. In that case, you can cut a small washer to insert into that cap out of rubber or nylon or similar material. That is usually not needed.


You can use the cap, but also wrap threads with Teflon tape as the threads are not tapered. The bag text is just generic text for all the fittings.

  • 1
    Teflon tape is normally not used on compression fittings. Tapered threads like NPT specifically call for a thread sealant such as tape, so I think you're saying the opposite of what you intended? The tape on the fittings in OPs picture doesn't need to be there and can actually interfere with the rubber seals in those braided lines.
    – JPhi1618
    Commented Dec 19, 2019 at 18:16
  • True compression fittings are straight threads either UNC or UNF depending on size and I agree teflon tape should NOT be used "normally" on those type of threads BUT he is using a cap that does not have a seal from what I can see on the image. Compression caps caps either come in 2 parts or 1 part with rubber seal, in his case based on what he has the tape will work, not the best solution but it will work. Commented Dec 19, 2019 at 18:48
  • @user1946891 - you mention that given the cap I have that tape will work, but is not the best solution. Is the best solution a cap with a flat rubber washer/seal inside? If I wanted the best bet should I go to other hardware stores and look for a cap that comes with a rubber seal? Also, you mention a cap that can “come in 2 parts.” One part would be the metal cap - what would the other part be? Thank you!!
    – Han
    Commented Dec 19, 2019 at 19:22
  • the 2 part ones I have used were on larger diameters the inner is like a plug and the outer is a ring. With the cap you have a simple piece of flat rubber washer would work Commented Dec 19, 2019 at 19:25

For posterity I wanted to answer this post with what worked for me. The info in the other answers about the confusing text in the package was invaluable as was the reminder that there should not be tape on the braided supply lines (they already have a washer inside).

I did want to follow-up on the advice that the cap did not need tape: without tape or a washer this thing leaked like crazy. Even with <5 wraps of tape this leaked. The only way I was able to get it to stop leaking was to add closer to 8 wraps of PTFE tape.

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