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For threaded plumbing connections, you typically use pipe dope or thread tape (aka teflon tape). Are there situations when one is preferred over another? Does the size of the pipe, type of pipe, part being connected, or other variable make a difference in the selection?

Note, this assumes you aren't using both at the same time.

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I saw something once with a max pipe diameter for plumbers tape. I'll see if I can't find it again. –  mac Nov 29 '12 at 22:13
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3 Answers 3

up vote 6 down vote accepted

Whether dope is required at all, is dependent on the type of threads. The type of thread is determined by the tap or die used to create the threads, and should be labeled on the pipe or fitting.

Thread Standards

National Pipe Thread (NPT)

This type of thread when mated, may contain slight gaps between the major and minor diameter of the threads. Because of this, a thread seal agent may be required. However, this type of thread is tapered at a rate of 1/16 (3/4"/foot), which often allows a seal to be made without a sealing agent.

May be listed as MNPT or FNPT, for male and female threads respectively.

National Pipe Thread Fuel (NPTF)

NPTF threads are designed so that when the threads are mated, they actually deform to create a mechanical seal. Since the seal is created by the threads themselves, a thread seal agent is not required (though may be used as a lubricant).

May be listed as MNPTF or FNPTF, for male and female threads respectively.

National Pipe Straight (NPS)

This type of thread is similar to NPT, except that the thread does not have a taper. A thread seal agent is required, but should be selected differently due to the lack of taper.

May be listed as MNPS or FNPS, for male and female threads respectively.

National Pipe Straight Fuel (NPSF)

NPSF like NPTF creates a mechanical seal due to the deformation of the threads, however, unlike NPTF NPSF threads are not tapered.

May be listed as MNPSF or FNPSF, for male and female threads respectively.

Selecting Dope

Which form of dope you choose is often based on who you learned from, personal preference, what's on hand, what type of pipe you're working with, etc. The following are my personal guidelines.

Water

Plastic, Soft Metals, Unreactive Metals

In this situation I'll reach for PTFE tape, since it's not likely I'll require long term corrosion protection. I'm just looking to lubricate the joint, so I can tighten it to create a leak free joint.

Steel, Reactive Metals

Because I don't want the threads to rust or react negatively, in this situation I'll use paste dope. The paste dope will give me the lube I need to get the pipes properly joined, while at the same time providing corrosion protection. The paste dope will never harden or flake off, so I know the threads will be protected for a long time.

Large Diameter Pipe

If I'm working with water pipe of any material larger than ~1", I'll always use paste dope. I don't really have any particular reason for this, it's just the way I was taught.

Fuel Gas

When working with fuel gas pipes of any kind, I always use a paste dope labeled for this use. This is especially true when working with "black" pipe, since I want some corrosion protection in these joints.

Tapered VS. Straight Threads

Tapered Threads

For pipes and fittings with tapered threads, I'll simply follow the guidelines mentioned above.

Straight Threads

When working with fittings with straight threads, paste dope should always be used. Tape dope is too thick, and can actually prevent a good seal in straight thread joints. Paste dope will spread and be pushed out of the way, and will not prevent the threads from properly engaging.

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If you're only going to use one or the other, I would use:

  • Metallic pressurized water fittings: dope because it will lubricate more effectively without being damaged by hard threads and you're at less danger of breaking the fitting due to over tightening.
  • Plastic pressurized water: tape because it will slide equally effectively against plastics, and will not "over lubricate" lending to breaking the fittings. Also, certain dopes can react with certain plastics (though most are probably safe).
  • Metal/Plastic drains: dope because it will more readily fill gaps to "seal" if there is no pressure backing it.
  • Air/Gas: tape (or specialized dope, or nothing at all, depending on the fitting) because you don't want to potentially spread oils or other dope through your appliances/tools.
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Could you expand on why you would use one over the other in each of these situations? –  BMitch Nov 29 '12 at 22:13
    
@BMitch edited for detail –  Matthew Nov 29 '12 at 22:44
    
And you don't need tape or dope on compression fitting threads (where the seal is typically against two tapered surfaces as opposed to "between the threads"). I've seen this done on more than one DIY TV show. –  Les Nov 30 '12 at 14:21
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Rule of thumb, even though I'm sure some will disagree:

Dope on course threaded pipe, galv, black pipe etc. Also on larger diameter pipes and always special oil resist dope on fuel. oil and gas (propane,natural gas) pipes.

Tape on small 1/2 and 3/4 inch tubing and plastic fittings. Tape is almost always used on most small brass fittings for water and air. Never use dope on plastics, strictly tape.

I notice that some of the old time plumbers tend to use pipe dope on more applications than newer plumbers. Not sure if that is because they think it works better or just that it is what they have always used before tape was commonplace. Now Pex and compression tools taking over.

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