I've got this piece in front of me that is off a locomotives whistle which has threads I'm not familiar with. These are tapered threads, but these aren't any designation which I can find. I've been told perhaps "Bridge Taper Threads" or "Steam Boiler Threads". Only thing I know for certain is this is for Locomotives which have steam boilers. This isn't NPT or BSPT nor is it NPTF. The outside of the largest diameter part of the thread is measuring pretty much exactly 2.500". I'm being told that there are 12 threads per inch, though I can't really verify that as there isn't really a full inch of threads going on here.
Boilers use ASME that in turn uses ANSI specs for threads
ANSI has specs for most things like this, but you have to pay for the standards.
Measure the threads over 1/2" and x2
One other possibility is the threads dont match anything. Both items may have been made by hand.
Vintage Steam power always warms a special place in my heart.
Do you know the origins of the locomotive?
could it be an M60X2.0T?