The straight threads you refer to are called NPS, they are the same dia. and
pitch as NPT but use an o-ring at the neck to seal. Valve to bottle
connections are where I usually see them such as scuba tanks, fire
extinguisher CO2 paintball or beverage tanks.
wrote in message
oups.com...
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