Standard threaded fittings as used in most irrigation and plumbing are very slightly tapered. This creates a more positive seal as the male thread is twisted into the female thread, to prevent leaks. So the diameter of the male threads increases slightly at the back of the threaded portion, while the diameter of the female threads decreases further inside the fitting. In most cases this taper is not sufficient to create a reliable seal, so a sealer is used. For irrigation systems the recommended sealer is PTFE thread seal tape (often called Teflon® tape.) Do not use “pipe dope” (liquid or paste type sealers) on irrigation systems! If it comes in a tube or bottle, don’t use it. Look on the bottom of many sprinklers and you will see the warning “Don’t use pipe dope”. This is not an insult aimed at people who use pipes rather than tubes! When pipe dope or paste works it’s way inside the pipe (which it will) the water will carry it to the valves, drip emitters, and/or sprinklers and clog them up and ruin them! So the sprinkler manufacturer is warning you not to use them. Only use PTFE tape type sealers. If tape gets into a valve or sprinkler it can be removed and the damage is not permanent.
Using PTFE Thread Sealant Tape (Teflon® Tape)
When joining male and female threaded fittings, put a nice thick layer of PTFE thread seal tape on the male threads before you screw them into the fittings. Pull the tape tight onto the male thread so that the tape molds into the threads. Wrap it in the direction of the threads so it doesn’t unwind off when you screw the fitting on. (If you are looking at the end of the male fitting that would be clockwise.)
How much tape to use? The old standard was “3 wraps”. However now they are selling low-cost PTFE tape that is thinner and requires more wraps. When you have enough tape on the male threads the shape of the threads will be just barely visible through the tape. I personally prefer to err on the side of using too much tape, it is not fun to find you didn’t use enough after the water is turned on and you discover a leak.
Connecting the Fittings
Once the tape is on the male thread screw the male thread into the female threaded fitting. If the joint is between two metal pieces put a wrench on it and tighten it as tight as you can get it. If one or both of the fittings is plastic just tighten it by hand. If you are an average guy or gal you can add one more full turn using a wrench after it is hand tight. If you have ever been called a gorilla for your strength or grip, stop at hand tight. Over-tightening plastic fittings splits the female fitting, resulting in a leak. But not tightening them enough also gets you a leak.
Avoid joining a male metal threaded fitting to a plastic female threaded fitting. This will be a disaster if you are not very careful. The male metal end does not give at all, and the female plastic fitting is likely to split open unless it has heavy reinforcement. If you must join male metal to female plastic, use lots of PTFE tape and hand tighten only. No wrenches!!
High density polyethylene (HDPE, trademark name is Marlex®), street ells often don’t need PTFE tape to seal, but I still use a little on them. It will not hurt to use a couple of wraps of PTFE tape on HDPE fittings. HDPE is a shiny, black, plastic that is slightly softer than PVC and feels slightly “oily”. HDPE is good for places where the fitting needs to be able to rotate and not “seize up” over time. Metal, PVC and PBS plastic threads will seize up and not turn easily once assembled. The HDPE fittings are idea for sprinkler risers where you want the threaded joints to remain pliable and able to move and absorb impacts. Warning: The black barbed insert fittings used to connect to flexible tubing risers (often called “Funny Pipe®”) are not HDPE. HDPE is seldom, if ever, used for any fitting with barbs. This is because the slippery surface of HDPE makes the tube slide off the barbs! If it has barbs, be sure to use PTFE tape on any threads.
Rule of thumb: If you can’t scratch the plastic with your fingernail, use PTFE thread seal tape on the threads!