HANDS ON HINTS FOR HOMEBUILDERS
NPT Fitting Installation Making sure connections won’t leak BY JOE NORRIS
EVERY HOMEBUILT PROJECT INCLUDES systems that require fluids to operate, the most obvious of which is the fuel system. Many aircraft call for oil, hydraulic, and possibly water lines as well. All of these systems will undoubtedly have some connections that will use National Pipe Thread (NPT) fittings. NPT fittings have a tapered thread on both the male and female side of the connection. The seal is actually a crush seal between the joining metal surfaces on the flank of the threads. The more you tighten the connection, the stronger this crush seal becomes. But how tight is tight enough? The short answer is “tight enough that it doesn’t leak,” but there’s a bit more to the process than that. Aircraft maintenance guidance often lists specific torque values for various fittings and fasteners, which works well most of the time. However, torque is not always practical with NPT fittings for a variety of reasons. For one thing, you often need the fitting to be aimed in a particular direction when installed, and tightening to a specific torque may not result in the correct orientation. Also, because of the wide differences in friction (material, sealant, or tape), torque may vary from connection to connection. As a general rule, NPT fittings should not be assembled to a specific torque value, because the torque required for a reliable joint varies with thread quality, port and fitting materials, sealant used, and other factors. So how do we get a connection that won’t leak? No matter how tight you make NPT threads, a leakage path still exists. Even in a completely tight connection, clearance still exists between the crests and roots of the adjoined threads, creating a spiral leakage path around the threads. For this reason, NPT threads are designed to be used with some form of sealant. It is the function of the sealant to block the path between the male and female thread. Another function of the sealant is to lubricate the connection against the
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galling and tearing of metal that takes place during the installation of the fittings. So what should you use as a sealant? The most common thread sealant is polytetrafluoroethylene (PTFE) tape, usually referred to as Teflon tape. While many mechanics say that it is unacceptable to use Teflon tape on aircraft systems, that’s not entirely true. Teflon tape is acceptable if used carefully and correctly, but there are better solutions. Many builders use Loctite PST Thread Sealant 592 with success, and there are other similar products on the market; just be sure your selected sealant is resistant to the type of fluid used in the system on which you’re working. When using tape, limit application to two or three wraps of the male threads. Using too much tape will increase the risk of contaminating your fluid system. Never put tape or sealant on or over the first thread of the fitting. This can cause the tape or sealant to easily enter the fluid system, which could damage the system components. Wrap tape in a clockwise direction when looking at the threaded end of the fitting. If not wrapped in the correct direction, the tape will not properly lubricate, potentially resulting in leaks. Make sure tape or sealant conforms to the threads, covering the thread crests. If the crests of the threads protrude through the tape or sealant, galling might occur. Be sure to remove all excess sealant and apply new sealant prior to reassembling a threaded connection after disassembling it for maintenance. Old sealant from initial installation could act as a leak point on subsequent assemblies. The bottom line is that tightening NPT connections is as much art as it is science. Using the proper sealant and having a good feel with your wrench will result in a leak-free connection.
PLAN VIEW The tapered threads form a crush seal when tightened against each other.
SPIRAL LEAKAGE PATH Clearance gap between the crests and roots of the adjoined threads.
1° 47’ inch
ILLUSTRATIONS BY ALAN LEPKOWSKI
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