Pipe Threading Lessons

Here’s a look at how to effectively handle threading-related issues on repair jobs

Pipe Threading Lessons

Anthony Pacilla

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If you need to screw a plastic fitting into an existing metal female adapter you must take great care to prevent cross-threading. Plastic threads are obviously no match for steel or copper threads, and forcing it and cross-threading will only lead to a ruined plastic fitting. Still, you should always have multiple plastic fittings on hand as cross-threading oftentimes is inevitable. 

Do not over-wrap the threads with Teflon tape. This will make it harder to start and eventually lead to a leak. And do not use a half bottle of pipe dope, then thread tape, then more pipe dope, etc. When the fitting goes through its “creep and relaxation cycle,” it will leak.

Threading Plastic Pipe

If you need to thread plastic pipe, you should remember that only Schedule 80 PVC and CPVC can be effectively threaded. Sure, you can thread almost anything, but these two are an approved method of pipe connections.

You must begin with a squarely cut pipe before you can attempt to thread. Try to use a miter saw or saw box to cut the pipe perfectly square and make sure you remove any burrs just as you normally would with steel pipe. You should be using threading dies that are manufactured specifically for threading plastic pipe. Do not use steel thread dies as you would with steel pipe. I would also suggest keeping it to hand-threading only. Power threaders can be too strong and not offer enough “feel” to thread your plastic pipe.

Overtightening

After cross-threading, overtightening is the second biggest mistake plumbers make while using plastic piping to make repairs. One quarter turn too many will lead to a cracked plastic fitting, and you starting over again.

An effective strategy is to take an iron nipple and screw it into the fittings first to help clear out any nicks or burrs that exist on the old female adapter. Another life saver is to take a tap, for example a 1 1/2-inch tapping tool, and run it into the fitting to clear out some of the debris, and then run an iron nipple in and out to clean things up. Even though it is not technically sound to use pipe dope on this type of connection, I have seen many successful old-timers use thick Teflon-based pipe dope on the threads instead of Teflon tape. It seems as if the Teflon tape is too thick sometimes and stresses the plastic, not allowing it to fully engage, which causes you to overtighten and ruin the fitting. 

Removing a Broken PVC Male X Steel Connection

Often in the field, you will run into a plastic fitting that has either been broken off, or cut to such a small exposure, that you can’t get a wrench on the fitting for removal. In this event you need to have a few tricks up your sleeve.

First, if you have an internal pipe wrench or something similar of an appropriate size that can produce an easy out, use it. This is by far the most effective way to remove a stubby piece of pipe from its female receiver. Get the appropriate size, gently tap it into the center of the broken fitting, and spin it out with a wrench. If you don’t own an internal pipe wrench, you should order a set that covers at least 1/2-inch up to 2 inches.

Second, if you run into a situation where you don’t have an easy out, you can modify a solution on the go. Say, for example, you are trying to remove a broken 1 1/2-inch PVC male adapter stuck in the side of an old 3-inch cast iron closet bend. You would get a 1 1/2-inch black iron nipple about 6 inches long, put it in standing up in a bench vice, and make a few long cuts downward from the flush portion of the threads downward into the threads about 3/4-inch. Now gently beat the threads inward until there is a slight taper and you now have a makeshift easy-out tool that may get you through the job. If you don’t feel like going through all the pain of creating such a tool, you can simply take a small one-handed hacksaw and carefully cut a series of relief cuts from the inside of the plastic pipe, being very careful about stopping before you start cutting into the steel threading. Then either pull the sections out with adjustable pliers or pry them out with a flat-tip screwdriver.

Lastly, you should always clean the steel female threads out with a tapping tool, or at minimum a steel nipple, just to keep everything clean and clear before continuing. 

About the Author

Anthony Pacilla is a registered master plumber for McVehil Plumbing in Washington, Pennsylvania. He has 23 years' experience in the plumbing and HVAC trades, and has a bachelor’s in business and economics from Thiel College. 



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