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Regarding NPT fittings, they are designed that way. There is a slight taper
built into the length of the threaded part of the fitting so that the thing
tightens up and seals as the male part is screwed (hmmm, could be a joke in
there somewhere) in. Due to the vagaries of manufacturing tolerances, it's
impossible to predict just how much engagement you'll get with any two
parts.

The big problem usually occurs when trying to align an angled fitting in
some desired orientation. The darn things usually get tight when you're 90
degrees from where you want it. I've found that using a good gooey thread
seal, like BakerSeal, will allow you to get that last 1/4 to 1/2 turn needed
by carefully and repeatedly turning the fitting back and forth a little
working it to the position required without getting that over-tightened
feeling. My guess is that this slowly wears the sharp or rough edges off the
threads allowing a bit more engagement.

For the fuel tank drain fittings, I actually did chase out the threads with
the proper pipe thread tap - but VERY carefully - going no more than a
quarter turn past tight and then checking the fitting's fit. It doesn't take
very much at all to go too far resulting in the impossibility of getting the
interference seal, so if you must do it, do a very small amount at a time.
If you get to the point where your male part screws in as far as the end of
the threads, you've scrapped the female fitting.

Dan Schaefer

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