That is what I was afraid of that the NPT 1/4-18 would be to big. Rats. The pipe nipple is the same as in steamwhistle photo in a few posts back and is attached here.
I have the pipe nipple in my caliper, though it is missing one side of the threaded pipe (it disintegrated) - so the reading shown in the caliper is a bit short. [ Guests cannot view attachments ]
Edit:I believe this die from eBay is what I need: Live Steam 1/4-40 ME Die - New Train Tool