DIYbanter - View Single Post

wrote:

snipped

Even on pipe threads....the pipe dope or tape doesnt seal...it
lubricates the tapered threads so they will tighten up and ground into
one another making the seal.

You are absolutely wrong about that, but you're not alone, many others
hold onto that mistaken belief.

Take a look at the design of an NPT thread standard in Machinery's
Handbook (or elsewhere) and you'll see that the crest of the thread is
truncated, causing it to leave a "spiral leak path" along the thread.
That path MUST be filled with some kind of sealant to keep the joint
from leaking.

Only a hack would try and overtighten NPT threads to force them to seal
without some sealant to fill that spiral path.

NPTF pipe thread (Also called "Dryseal" threads)are used in some
specific applications for fuels or solvents which might destroy pipe
dopes. Those threads are usually on brass parts and are designed with
sharp crests which crush when tightened to create a metal to metal seal.

Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."