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I removed what I didn't realize was a compression fitting on a copper pipe for my kitchen cold water supply. Is it possible to reuse the compression fitting or do I need to buy a new one to replace it?

It looks like it's some kind of plastic. It took a tremendous amount of force to loosen, and a fairly cursory attempt at re-tightening resulted in a leak. Also, I assume I should wrap everything in teflon tape when replacing the connectors?

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    Teflon tape will be worse than useless here. Clean well without scratching. If you have an urge to apply something, somewhere, put a little bit of pipe dope (teflon pipe dope if it makes you happy) on the male threads - no effect whatever on the seal, but it will help the next time someone needs to unthread the thing (like anti-sieze, but safe for use around drinking water, which anti-seize is not.) – Ecnerwal Dec 10 '13 at 4:23
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    If it was me, I'd cut the pipe and install a T to run a proper valve for the blue line (ice maker?). Those puncture valves are prone to clogging and leaking, so you may as well fix it right while you're down there. Then after the T, install a new compression fitting for the sink. As others mention, the ferrule is deformed by the nut to make a water tight seal and won't properly seal if it becomes too damaged. – BMitch♦ Dec 10 '13 at 14:07
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    @BMitch it's to a basement RO filter, and yes, that's what I ended up doing after writing the post. It didn't need to be cut; I just disconnected the other end of the copper line that was attached to the cold water shutoff valve. I have no idea why my Dad presumably installed the copper line rather than pick up a tee when he hooked this up for me years ago. I've experienced problems with those valves in my basement humidifier connection. – glenviewjeff Dec 10 '13 at 14:38
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If it hasn't been tightened many times, it should be okay to reuse.

Clean off the mating faces thoroughly and don't use tape, putty, or anything else. And don't be gentle tightening: The seal is formed by forcing the surfaces together and "bending" them to mate.

Even with threaded steel pipes, it can take a day or two for the tightened fittings to form a good seal and they can drip at first. The essential observation is whether the drip rate slows hour by hour—if it does, it is probably okay and going to seal.

If it doesn't drip more slowly, try tightening a little more with water pressure applied until two consecutive small turns (1/8 turn) doesn't immediately decrease the drip rate.

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    I don't understand when tape is appropriate vs. dope, or nothing at all. Could you clarify? – glenviewjeff Dec 10 '13 at 15:01
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    A compression fitting is designed to make a tight seal in the absence of any tape/dope/putty. Your more general question about when these are appropriate would be a good subject for another diy.sx question – mac Dec 10 '13 at 15:06
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    @mac just did, thanks for the suggestion. diy.stackexchange.com/q/36613/9750 – glenviewjeff Dec 10 '13 at 16:01
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I would be cautious reusing this. Reason being if the surface has a slight indentation from the previous seat from the prior install, and the shape of that end is kind of half spherical, which allows the mating surfaces to change locations depending on the angle to pipe approaches its counterpart.

If the surface is not marred from the prior install though, it is safe to use. Perhaps even a slight mar may be reformed when tightened, worth a shot to me if that is the case.

Do clean it and inspect it first, apply pipe dope if you choose, not teflon tape

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  • I don't understand when tape is appropriate vs. dope, or nothing at all. Could you clarify? – glenviewjeff Dec 10 '13 at 15:02
  • The only place tape is to be used is on threads, to be exact, tapered threads, the kind that are on pipe nipple, male threaded ends of valves. If you look at the threaded end you can see the pipe is smaller at the end than farther down the length. Pipe dope can be used on these threads too. Pipe dope, otherwise can be used on the various types of compression fittings, the type you show here, the type that use a ferrule too. When using metal pipe the type of pipe dope is not that critical, but if using plastic, be careful, there is pipe dope NOT for plastic. – Jack Dec 10 '13 at 15:17
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    In a nut shell, tape for threads only, dope for mating surfaces, flat, smooth or rounded as yours. It is mainly insurance in my opinion, and I use it too, but the mating surfaces should make a tight fit on their own. I have had a many a leak before using dope, now the leaks are non-existent. Then again, maybe I just got better at it, finally.... – Jack Dec 10 '13 at 15:23
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I'm going to directly contradict the other answers. As stated in "See Jane Drill episode on "When to Use PFTE Tape, Pipe Dope or Neither" (see 6:06 onward), warping your ferrule with some plumbers tape is the preffered way to reuse a ferrule you can't remove.

As others have stated, the ferrule is usually scored during the initial fitting and that scoring will likely make it leak during reuse, the teflon tape workes by filling the fills the scored area of the ferrule.

That said, the preferred thing do to is remove the ferrule if you can. If I were in your situation, I'd cut and replace with a 'push fit' hose + fitting combo (if your valve needs to be replaced your hose should be too, so you might as well buy them as a combo) as they are easy to remove later.

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