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
galling and tearing of metal that takes place during the installation of
the fittings.
60°
P
PLAN VIEW
The tapered threads form a
crush seal when tightened
against each other.
1° 47’
SPIRAL LEAKAGE PATH
Clearance gap between the crests
and roots of the adjoined threads.
inch
