In article , Ed Huntress
says...

It looks very nice, Stu. About the strength issue: Pipe threads are made to
seal against fluid leaks, and are nowhere near as strong as a straight
thread, because first one wall (say, the pipe) and then the other wall (the
fitting) tapers to a thin section. So you only have real thread strength
over a short section near the middle of the thread.

NPT threads are pretty much always sharp V form threads at
the root of the male threads. This, along with the generally
rough surface finish in black iron pipe, will cause failures
to occur at the distinct stress concentration at the root
of the threads.

I would suggest that the joints in question be static
tested for, say, five or so times the static load they
are expected to bear in use, to account for any dynamic
loading they might see.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================