what (not) to use on fuel line fittings

[Garfield Willis]

On Sat, 27 May 2000 19:19:42 -0400, Chris Conlon <synchris at ricochet.net>
wrote:

>At 10:37 AM 5/26/00 -0700, Garfield Willis wrote:
>
>>BTW, one last thing, teflon tape is NEVER used on aircraft fuel
>>plumbing. Just NEVER, for the very reason you cite above.
>
>I've been using Permatex 14A (liquid teflon sealant), with no ill
>results so far, but I don't muck around with fuel fittings much.
>Is this gonna bite me? Is there something else I should switch to?

No, that's fine, as long as you start a couple threads before slathering
up the rest of the threads. That prevents the stuff from ending up on
the tip of the thread that makes it thru the taper. What you want is the
sealant to dam up in the several last voids of the threads being
compressed in the pipe taper. [I'm assuming you're talkin bout pipe
thread, cuz if you're using AN fittings, you don't USE any sealant at
all, which is of course the absolutely preferred way, but often you're
stuck with a pipe thread adapter transition even to AN's].

The reason why Teflon TAPE is a no-no is manifold (rhymes with twofold).


First, many people ignorantly wrap it all the way out to the nose of the
male thread, and much of this may get sheared off when the pipe thread
taper compresses, leaving you with a shard of the tape lose on the
inside end of the male portion. You wouldn't believe how extremely
common this is. IF you could manage to fit up the joints so you KNEW
where the taper was going to be absolutely all inside the female
threads, then you *might* be OK to tape only that portion of the male
threads (no harm can come since you've already ascertained that that
portion of the threads will not be exposed inside, via your previous
fit-up exercise).

BUT, here's the second problem, even if the first were handled. Spose
you then have to remove such a fitting/open the joint? You've got
compressed tiny threads of teflon tape, which don't particularly care to
adhere to the inside threads once you remove the compression of the pipe
thread taper. Can you be sure you get all those out?

Finally, the worst problem with teflon TAPE is that the nature of the
wrapping followed by the thread compression cuts it into long
thread-like shards, that IF they should get loose, VERY easily
clog/occlude especially small orifices (can you say FI or jet
orifices?). The fact that the crap ends up in long thin shards makes it
especially problematic AND they may linger for a long while hung up
somewhere before letting loose and clogging a critical, small artery
like the aforementioned orifices. Almost like a lodged blood-clot causes
further clotting, if the thread should get hung up in a very small
opening, it then tends to dramatically ball up and suddenly the clog
isn't just a restriction, it's a blockage.

The liquid ptfe sealant has just as many risks for occlusion as any
liquid plumbing sealant, but it *doesn't* have the thread-like
characteristic of the tape, AND it tends to stick in crannies, and when
it doesn't, breaks up into particles instead of long spindley threads.
When you pull apart a join previously sealed with liquid teflon sealant,
it tends to crumble into particulates, rather than long threads. This is
bad if it gets into the system, but if you're going to get a blockage,
it's going to show up sooner and more noticeably than a thin thread of
stuff worming it's way around your system, looking for the smallest
possible passage to then clog into.

The VERY best way to avoid stuff like this is to use as many and as
completely thruout the system as you can, AN-fittings. The nature of
these fittings precludes the need for sealants. And where you per force
have to seal a pipe thread interface, remember that the nose of the male
portion often doesn't get locked up in the taper, unless the female
portion if deep enough to take it all. So think about where your sealant
might end up once you screw the threads in home, and especially keep the
nose of the pipe thread completely void of any sealant. Cuz if it
doesn't get compressed in the pipe taper, it's gonna come off sooner or
later and move around your fuel system.

Here endeth the sermons I too was given upon entering the sport of
experimental aviation. You'll excuse the sermonic tone; it's passed
along from generation to generation. Heh. I'm sure something similar is
whispered around many auto racing circles, but I expect the 'religion'
isn't quite as cloistered and uniform/orthodox as in XAv circles where I
got started in all this engine crap, so YMMV.  :)

HTH,
Gar


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