On 7 Feb 2005 09:06:27 -0800, wrote:

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.

The same threading problem occurs with metric pipe threads, excep that
each country with metric standards has it's own standard. That is why
there is a 1/8" pipe tap in my set of metric tape/dies. Many Asian
countries use the American pipe standard.

I wonder if the shop doing your work happened to use the parallel tap
instead of a tapered tap?

Paul
Greetings Paul,
NPT does not refer to both straight and tapered threads. Straight
pipe threads have the following designations: NPTR, for railing, NPSC,
for couplings, NPSM, NPSL, and NPSH, for different types of mechanical
joints. Along with the preceeding are NPTF,PTF,NPSF, and NPSI, all
different designations for dryseal pipe threads.This information comes
from Machinery's Handbook, 26th edition, published by Industrial
Press. (gotta give credit where it's due).
Cheers,
Eric R Snow