The starting diameter should be 0.405" (10.23mm). The thread, being tapered will get progressively harder to tap as you get further on. The angle of taper if you intend to use a lathe, is about 1.5 degrees.

I've never cut an external NPT thread but have tapped them. As you'd expect the tap quickly gets more difficult, followed by impossible, to turn. The tapping drill tends to be slightly larger than one might expect. I expect a die would be similar. I'm not sure if a taper on an external item would work. The die will go on further before it cuts, but I suspect it'll come to a halt more quickly. So possibly not much gain.

This is how I cut external tapered threads:

I was screwcutting BSPT threads but I think they're the same taper as NPT. I thought NPT threads were specified by a nominal imperial dimension representing the pipe size rather than tpi?

Andrew

Now now, Roy, you could get a tap on the head for that (and water on the brain).

The plug gauges for pipe taper threads have a flat below the rear surface and the rear surface to act as limits for threading. It is surprising how few threads are enough to be classed as acceptable.

Picture of a taper thread gauge

Martin C