Stripped threads in carburetor

Mike Romain wrote:
> ray wrote:
>
>>Mike Romain wrote:
>>
>>>
>>>I realized that as soon as I hit send and was expecting the above.
>>>
>>>Daniel is correct, for any fitting with a sealing face or o-ring trying
>>>to seal the threads is a waste of time.
>>>
>>>Even with some tapered fittings, teflon isn't allowed by safety code.
>>>Gas pipes need liquid pipe dope and we use it (dope) for hot water
>>>heating pipes.
>>>
>>>Mike
>>>86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
>>>88 Cherokee 235 BFG AT's
>>
>>When I installed my nitrous system the instructions told me in no
>>uncertain terms NOT to use teflon tape on AN style fittings and use
>>teflon PASTE on pipe thread fittings. (In this case it's to prevent
>>teflon tape from plugging a nitrous line.)
>>
>>I find that kind of neat working on cars - there's
>>fine/coarse/flared/pipe thread fittings - metric/standard, and I'm sure
>>more. Occasionally that's a pain, but it's still interesting - you
>>learn all sorts of new skills.
>>
>>Ray
>
>
> You are right. That tape can also gall up so the threads don't end up
> with a good metal to metal seal. This causes slow leaks eventually when
> the built up pieces squash from vibration finally and the fitting comes
> loose.
>
> The poster was asking about air line fittings, those are a compression
> tapered end if I remember right so the threads have to go on really
> tight to seal the metal nipple to the cone. You can ball up enough
> teflon tape to make a seal, but it will eventually leak, the least
> amount that you can stick on is the best so the metal to metal at the
> end tags. It is a lubricant. Teflon over the seal face end will
> eventually fail or plug things.
>
> Mike
> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
> 88 Cherokee 235 BFG AT's

I hope you will not take this the wrong way Mike, but I can't tell if
you fully appreciate the design principles of different kinds of tapered
pipe threads.

Take a look at the thread form of NPT threads in Machinery's Handbook
and you'll see that the crests of the male and female threads don't come
to a sharp point, but are flattened. The roots of the threads are closer
to a sharp point, so the crests don't fully fill the roots. This is done
on purpose to make sure that the flanks of the threads will contact each
other to support axial loads on the joints.

That flattening of the crests of the threads leaves a small trapezoidal
shaped path spiraling along the full length of properly torqued engaged
threads, sometimes referred to as a "spiral leak path". It's that path
which has to be filled up with some kind of "pipe dope" to keep it from
leaking, and the choice of what dope to use is yours to make, I still
prefer using Rectorseal or a paste type dope for water service.

NPTF threads are what's called "Dryseal" threads. Those threads have a
sharper crest on the male threads and are designed to create a metal to
metal crush seal along the spiraled thread path, without needing any
sealant to keep them from leaking. They are more often found on brass
fittings, where the mallability of the brass makes it easier for the
crush seal to occur.

The last letter in NPTF, the "F" stands for "fuel". When the thread form
was originally developed it wasn't easy to find pipe dopes which would
withstand fuels, to a thread which would seal without and dope was
designed.

A few lines swiped from Machinery's Handbook which describing NPTF
threads a

++++++++++++++++++++++++++++++++++++++++++++++++++ +

American National Standard Dryseal Pipe Threads for Pressure-Tight Joints.

Dryseal pipe threads are based on the USA (American) pipe thread;
however, they differ in that they are designed to seal pressure-tight
joints without the necessity of using sealing compounds. To accomplish
this, some modification of thread form and greater accuracy in
manufacture is required. The roots of both the external and internal
threads are truncated slightly more than the crests, i.e., roots have
wider flats than crests so that metal-to-metal contact occurs at the
crests and roots coincident with, or prior to, flank contact. Thus, as
the threads are assembled by wrenching, the roots of the threads crush
the sharper crests of the mating threads. This sealing action at both
major and minor diameters tends to prevent spiral leakage and makes the
joints pressure-tight without the necessity of using sealing compound.

++++++++++++++++++++++++++++++++++++++++++++++++++ +++

All that's well and good if male and female threads are manufactured to
specs, something that doesn't always happen, particularly with some of
the cheap imported junk we're seeing nowadays.

And, I do agree with some of the things you have to say about teflon
tape as a sealing material for NPT threads. It's not always easy to make
sure the teflon squishes where it should go to completely fill the
spiral leak path. And backing off a joint a bit after proper tightening
and then retightening it can displace the teflon so the joint won't
seal. That generally doesen;y happen with paste type dopes while they
are still "fresh".

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

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