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When is it appropriate to use tape, dope, or nothing at all when connecting pipe fittings? Several advised not to use any product when I posed this question. What is the purpose of each product, and what problems would they solve or could they cause?

To preempt duplicate question flags, this question is focused on tape vs. dope, and doesn't ask about case where neither is recommended.

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The answer to the second question you link to mentions when dope/tape is not required. Though the answer only focuses on threaded connections, since dope/tape is only used on threaded connections. –  Tester101 Dec 10 '13 at 19:39

2 Answers 2

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The only place tape is to be used is on threads, to be exact, tapered threads, the kind that are on pipe nipples, or the male threaded ends of valves for example. These are the kind that get tighter as you twist the parts together. 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 that use a ferrule. 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.enter image description here

In a nut shell, tape is for tapered threads only, dope can be used too. and either must be used or a leak will occur. Dope is to be used only on mating surfaces, flat, smooth or rounded. 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....

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As a mention, there are three kinds of pipe dope that I am aware of, good for metal and plastic joints, good for metal joints only, and the type that is used for gas line fittings whether they be for propane or natural gas. The type for gas lines is usually recognizable by the yellow labeling, the color coding for gas lines in construction. –  Jack Dec 10 '13 at 19:46

Tape or dope should be used when the threads of a pipe are being used to create the seal. In a compression fitting, the ferrule is what creates the seal by deforming to the shape of the pipe. Similarly, some gas pipes use a flare fitting to make air-tight the connection.

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