I fixed a set of compressed air leaks today. An entire set of plugs I purchased all leaked. (I suspect they had the right nominal threads but weren’t tapered properly; NPT connections should always be tapered.) But I suspect there may still be non-trivial leaks around. If you see evidence of any, submit a problem report to let me know.
Dang someone sells NPS thread? That is quite difficult to buy. I recently had to buy a NPS die to cut threads for a hydraulic press fitting.
It was listed as NPT…. I’m just guessing why they sucked, but they definitely did.
Unlikely they’re improperly machined.
It sounds like you’ve got a mismatch of standards- NPT/NPS (60 deg tooth angle, flattened tooth peak and valley, and if NPT 3/4in per foot taper)
and BSPT/BSPP (aka “G”) (55 deg tooth angle, rounded tooth peak and valley, and if BSPT, same 3/4” per foot taper).
NPS and BSPP are straight.
The TPI matches only for the 3/4” and 1/2” sizes. The others have differences in TPI, but so small that it would be very difficult to see the difference. But they won’t mate up for many turns at all.
But even where the TPI matches, the difference in tooth angle and peak/valley finishing means these two standards won’t make a hydraulic or pneumatic seal, even with PTFE tape
That’s on top of whether you got the straight version of NP/BSP.
It’s insidious. It’s really difficult to say which standard you have without a microscope.
Every component purchased was clearly described as NPT. So if one set of items doesn’t fit properly, it was improperly machined or improperly labeled.