In article .com,
says...
A few months ago I needed to make a similar manifold and investigated
pipe taps for the project. I discovered that "NPT" can refer to either
tapered or parallel treads. A 1/2" tapered will screw into a 1/2"
parallell, but not seal. The opposite is not true. The parallel thread
requires a sealing mechanism such as an "O" ring.


No, NPT is a complete, non-ambiguous specification. There are a few
straight pipe threads, and variations on tapered pipe threads, but only
one NPT. Here's a chart that looks pretty inclusive...

http://www.parker.com/ead/cm2.asp?cmid=3562

Ned Simmons