What do you have? You have a fitting that looks like it fits and a clean cut. You tighten it down by hand until it won’t freely turn anymore and then give it two to three additional turns with your wrench. You think the job is done? You turn on the water and notice there’s a small drip coming from where they are connect.
This is frustrating and all to common for many folks when it comes to tapered pipe threads. NPPT threads are designed to mechanically deform to create a seal, provided you use the correct geometry. Unfortunatley most folks think about pipe fitting as plugging something in like an electrical cord. Push it in and hope for the best, that doesn’t work.
Why Your Pipes Leak
As you can see in chart above, thread engagement depth depends on size; the deeper the thread goes into material, the better it forms a seal without stripping the material out. So the important thing to understand: Small fittings is delicate. They don’t take much torque but are easily over-torqued. Large fittings takes more torque to deform the threads properly, but have some wiggle room.
Bottom line, hand-tight is only the starting line. This ensures you’re seated squarely on the taper. If you start cross-threaded here nothing you do with sealant will help you later on. After that, you’re in the wrench phase, two to three more turns to go until your fingers won’t turn anymore. Any less, and there’s a gap into which pressure can escapes. Any more, and you run the risk of stripping the threads on plastic component or even cracking brass fittings.
Remember: this isn’t a matter of muscle power. This is about geometry. The taper forces the male and female threads together radially as you rotate them. Every degree of rotation presses the flank of one thread against the flank of the other. Two to three turns with a wrench compresses just enough material to resist vibration without allowing flow.
I know folks can get crazy about thread sealants, globbing on pipe dope with multiple wraps of tape like they’re building a fort. The whole idea of that is a misunderstanding off the system. The sealant lubricates the beginning of the thread engagement and fills any microscopic imperfections in the threads, but does not take place of mechanical pressure. Use two to three wraps of Teflon tape in clockwise direction. For compound, simply coat the male threads equally. Don’t allow your sealant to collect at the end of the pipe, this will act as a shim and cause the fitting to not seat propery. Make sure you close the joint flush.
Keep in mind as well that these are nominal sizes. There is manufacturing tolerances. This means that although the chart offers exact length references, you must rely on visual checking and feel to confirm installation in field. Locate where the threads end and the fitting begins. When you get to that vanish point stop. You’re good.
It’s always a good idea to do a quick pressure test of your handiwork before you close the wall or cover the line. A little drip today becomes a wet basement tomorrow.
But pipe fitting is an exercise in discipline (not force). It’s a matter of starting straight, stopping on the last turn where geometry tells you to quit and doing your sealing job correcty. Nine times out of ten that slow drip in the beginning is because you started off crooked and skipped those last couple turns with the wrench. You should of trusted the turns, respect the taper and let the threads do their thing. You’ll have a good joint. The pressure will stay in, and you can walk away knowing you did it right.
