[jakewins]

Do you have a reference for this? Everything I've read is the purpose of the tape is to reduce friction (hence PTFE), not to actually seal. In other words, the seal comes from the fittings connecting tightly, made possible by 2-3 wraps of PTFE low friction tape.

[berjin]

A reference? This is plumbing not academia. The parts I get might be slightly different than the spec so you have to make up for it with the right amount of tape in situ. If you tighten the threads as much as possible and there is still a gap then you have to seal it somehow. Soft plastics will deform enough to seal without any tape at all but harder materials need something between to fill in the gaps.

[XorNot]

PTFE tape is meant to block the helical path around the screw threads for leaks to propagate. For parallel threads it's absolutely vital.

Taper threads are designed to crush together to achieve something similar, but for what's available at the hardware store I've always had problems.

[lazide]

That is a common falsehood.

Parallel threads need an alternative sealing method, or they’ll leak. If low pressure, dope or tape might be enough, but it’s using the fitting wrong.

Usually there is a flare, mating surface, or o-ring type setup that should be used instead. Examples would be welding tank connections, or scuba tanks that are at thousands of PSI.

For NPT, if the fitting is properly tightened, the fitting deforms and the threads mash together. They won’t leak even at hundred of PSI.