In article , Ed Huntress says...

Then the job is to quantify strength of a joint that was never designed for
much strength. Pipe threads are designed to seal, not to exploit the
strength of the material. Straight threads are.

And therein lies one problem. NPT threads want to get as close
to a zero clearance fit-up as possible, so the roots of the
male threads are machined sharp V. This provides a stress
concentration that would be atrocious in a fastener. Bolts
typically have rolled, not cut threads, and in the higher
strength grades have radii at the roots.

Still Ned's analysis shows it will probably hold up, as
long as nobody jumps on it or anything.

Jim

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