Did a Google search and it stated NPSM ( or American Standard Straight Mechanical Pipe Thread ) and they do not have a taper pipe thread. Type NPS stands for National Pipe Thread and have a tapered thread. When I started as an apprentice our electric shop had an old set of 1/2 to 2" Rigid stock & dies that only the sparkies were supposed to use. A know it all plumber asked to borrow them and I tried to explain that I was told they were not for use on plumbing pipe. Of course he told me that I was another dumb rookie. He threaded at least halve a dozen pieces of schedule 40 pipe for a 125# steam line. He spent hours taking first leaking joint and applying more pipe dope, put it together then tried fixing next leak. End of day somebody in our shop that heard our conversation told him he used the wrong dies. These dies did not produce a tapered thread and hopefully no longer produced. Came across European 2 to 5 KW immersion heaters that had straight threads. Some how it was the electricians job to remove bad heaters & wrench in new ones. Luckily we had a very talented chief electrician and he finally told us how to stop threads from leaking water. After we applied several layers of Teflon pipe tape wrapped two pieces of sewing threads onto taped threads. These heaters were on melting kettles and little pump only produced 10# of water pressure.
