The tapered thread seals use an interference fit between the male and female threads. NPT fittings should never bottom out in assembly. By hand assembly only, the pipe or fitting should only screw in part way before jamming. If an NPT pipe or fitting screws all the way into the female port without binding, the threads are either incorrectly tapped, worn out, or the tapers are grossly mismatched.
Within this specification, only specific line classes are permitted use of threaded piping, with some permitting up to NPT 2. ASME Standard B1.20.1 Pipe Threads, General Purpose identifies the required thread length engagement for hand tight plus wrench tight positions during assembly.
It is important to understand that torque is not the appropriate assembly technique (i.e., "Tight is not right") for NPT assembly, but rather thread engagement by the number of turns. Torque is not an appropriate method to makeup connections due to the differences in thread surface roughness, variance in threads and taper, lubricity of thread compounds, and other factors.
Fit to Hand Tight (FTHT) is commonly referenced in threaded piping assembly as the hand tightened position, without compound or tape applied. Table 11 below identifies the intended thread engagement in hand tight and wrench makeup for NPT 1/8” to 6”.
This procedure calls for a “dry fit-up” of the pipe and fitting to the L1 position during assembly.
Step One: From the position of FTHT the technician should count the remaining threads on the pipe, as well as mark the pipe and fitting on the axis of the pipe, to establish dry fit position.
Step Two: Following application of thread compounds and/or tape, the pipe connection should be returned to the hand tight position then further tightened by wrench the additional turns as prescribed in the ASME Standard for proper thread engagement.