what (not) to use on fuel line fittings

Greg Hermann bearbvd at cmn.net
Sun May 28 03:27:05 GMT 2000


>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.

Bingo. this is what I meant.
>
>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!

Precisely the spot where straight pipe threads are most appropriate! Or, if
the dweeb who tapped the casting with a taper pipe thread can't be
educated, put a pipe nut meant for use on straight pipe threads onto a
tapered thread fitting with an appropriate O-ring to seal (and lock)
things!

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.

Absolutely--a seal weld to cast iron ISN'T! Gotta have malleable or forged
pipe fittings to do seal welds. Wherein lies another nice little detail. If
we are being truly picky, only forged or billet fittings should be used on
things like fuel systems! Malleable, but not cast, iron fittings are
acceptable, per building codes, for fuel gasses up to 5 psi--but that
doesn't mean I have to like them!

Greg
>
>>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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