what (not) to use on fuel line fittings

[Garfield Willis]

On Sat, 27 May 2000 20:02:20 -0600, bearbvd at cmn.net (Greg Hermann)
wrote:

>There is a place for tapered pipe threads. It is in joints that are not
>expected to be disassembled, EVER!! Sealants are OK for non-critical stuff.
>If using tapered pipe threads in a critical system, put them together DRY,
>and use a SEAL-WELD to seal them!! (Like I said, they are really only to be
>used where disassembly is not expected to happen anyway.
>
>Which is precisely why socket weld fittings were developed--if you are
>going to weld it anyway, why bother with the threads ??!!

I'm not familiar with the "SEAL-WELD" you mention above, Greg. What is
that? Or are you just referring to thread-locking compounds of the
LockTite type? Last possibility I can think of, is you're suggesting
screwing in the threaded joint dry, and then welding it up! If so, see
below.

I know about socket weld fittings (basically they're raw fitting you
weld on), but unhappily, most of the time that you're stuck with a
tapered pipe fitting, it's when you're coupling to some casting that's
been drilled and tapped, and with many cast alloys, it's often not an
option to weld on a permanent fitting there. And if you DO try a weld
there, damn are you committed! You'd better be good at welding cast
alloys! Last but not least, whether or not the threaded part is from
billet or a casting, it's often part of an assembly that couldn't safely
take the heat of a weld anyway. I'm thinking specifically of things like
fuel-pumps that often come with fem tapered pipe threads. Even those
that are machined from billet are already assembled and dicey to try a
weld onto, for fear of damaging the innards from the heat. Oh sigh.

>Another, all but forgotten, alternative for such locations, or for use
>where one is changing from pipe stuff to AN stuff, and where sealants and
>welding are both unacceptable/impractical, is  STRAIGHT pipe threads! These
>have the same diameter (without the taper) and pitch and range of sizes as
>tapered pipe threads, but rely on a lock nut and an O-ring  for their seal.
>Straight pipe threads are perfectly acceptable for periodic disassembly,
>they are very vibration resistant, and no sealant is required! O-rings can
>be selected for proper compatability with the process fluid.

OK, yet another possibility!

Gar


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