If you’ve ever been in a hardware store staring down a wall of threaded fittings, then you know what I’m talking about. Why won’t that fitting screw onto your pipe? It seems to fit, yet it just never seals right. No matter how tight you can get that wrench, before long, it cross-threads and strips out the metal.
Chances are it’s not because you’re doing something wrong. It’s due to confusion about the function of National Pipe Tapered (NPT) threads. Unlike typical machine screws, which use a flat gasket for sealing, NPT threads actualy deform slightly as they mate. This form a type of wedge seal between metal flanks of the actual thread. The tapered action forms a watertight union The threads is sloped at a rate of one and forty seven minutes for each inch of diameter. You can see this in diagram in the next section.
How NPT Threads Work
As the male part meets the female part, the whole thing gets tighter. Not only does it create a mechanical seal, but it’s also an interference fit, it keeps water from leaking out. It’s not just a matter of two things rubbing together; it is a mechanical seal.
That’s important, because you can’t count on tightening something down until it stops leaking. If you crank too hard you’ll crack plastic PVC fitting or split cast iron fittings. The seal occurs at the sides of the threads. You’re simply adding stress without increasing security. DIYers and most plumber know better: a few turns with your hands first, then just a precise turn or two with a wrench. Here’s how it breaks out on the chart (above), in terms of how long you should engage each size of pipe.
To put it simply, though, here’s the practical lesson from the math: You need enough thread engagement to hold against the interior pressure. Why? So the weakest link in the chain isn’t the threads themselves. Typically, that means hand-tight, plus another turn or two, two turns if it’s a small line, such as three-quarter inch or half inch. More than that is probably overkill. You’re looking for snug, and you also want the sealants to do their jobs. It is not about using sheer brute force torque.
While the taper itself doesn’t close each tiny space inbetween thread grooves, there are several other considerations here such as the type of sealant used. For your air and water lines, Teflon (PTFE) tape is ideal. It will fill the gaps and also work as a lubricant during assembly. Because pipe dope stays pliable, it’s better suited for high-temp/high-vibration applications. In those cases, use pipe dope. It’ll adhere better to the threads with added pressure, yet remain pliable.
Some pros double-up for extra protection on things like petroleum or steam lines. That sounds reasonable considering how devastating those leaks can be. However, no matter which one(s) you choose, always apply sealant to the male threads only. If you coat the female threads, this drives the tape into the groove. Over time, that tape may separate and block valves downline. Now THAT’S a nightmare. Much more so then a dripping faucet.
A final note about straight pipe threads vs. NPT: You cannot interchange them. At first glance, they appear almost identical. The difference is in the sealing. Straight pipe threads requires a secondary sealing surface (washer, O-ring, etc.). With tapered threads, the sealing occur from the threads alone. If you use one where the other belongs, you have a gasketless leaky mess no matter how many layers of tape you apply. This is a major cause of returned product and anguished home owners. Because the packaging looks so much alike, they purchase the incorrect fitting. A simple pitch gauge and thread profile inspection will spare hours of aggravation.
Pipe threads require precision rather than strength. Understand the taper, apply the proper sealant, know how far to turn. Treat it like a precise assembly and leaks won’t happen often. Maintenance will be easy. When handled gently, the wedge stays wedged. It proves that gentle precision work better than aggressive force every time in plumbing.
