"Jeff Wisnia" wrote in message
...
Grant Erwin wrote:
carl mciver wrote:

I've come to understand that the tape is not for sealing. It is to
keep
the threads from corroding together and allow for removal. Having
recently
taken a steel fitting out of an aluminum casting, still holding the
aluminum
threads, it makes perfect sense.


OK, check this out. Take any pipe joint you want, wrap it in teflon tape,
thread the joint together as tightly as it would be in real life, then
take
it apart and see ALL the teflon tape gone from the joint. Unless it's
wiping
magic invisible teflon oil onto there, it won't prevent squat for
corrosion.
Tapered pipe threads seal by crushing together.

No they don't, not NPT threads at least, if they are made to spec.

Check the thread form drawings in Machinery's Handbook and you'll see that
the crests of male NPT threads are truncated, which creates the well known
"spiral leak path" requiring some kind of "pipe dope" to plug it.

NFTF ("Dryseal") threads, available on some small sized brass fittings,
have a form which provides the slight crushing action you refer to and are
intended to provide a tight seal without dope.

The "F" in NPTF stands for "fuel" as those threads were originally
designed to be used on fuel fittings long before Teflon or other fuel
resistant pipe dopes were a twinkle in some chemist's eye.

The teflon tape just helps
it slide in there a little. Teflon pipe dope doesn't really stay in there
either, but it is my opinion (and those of several pro plumbers and
pipefitters
I know very well) that it comes much closer than teflon tape.

GWE

Jeff (Who remembers using cotton string to seal pipe threads in his
earlier days.)

Still the way to go if the pipe holds water. Lampwick and dope if it's a
fitting that would be hard to redo, e.g. in a boiler manifold.