I got fed up of waiting for a tapered reamer I ordered from ebay to arrive and decided to make a start on the tapered nozzle without it.
I started with a 500mm long bar of 22mm diameter Delrin:
I then cut a 120mm length of this, and turned it on the lathe to allow me to cut 3/8" 19TPI BSP thread onto it.
Delrin machines beautifully. I'll be using it more from now on.
I had an unexpected problem when I can to use the 3/8" die I bought. The die holder I have is 1 1/4" but the die is 1 1/2". I looked through my stash of threading tools but I have nothing bigger.
I solved this by holding the die in the lathe chuck and turning the chuck with the chuck key. I was able to hold the Delrin rod by hand to keep it straight as the thread was cut.
I then applied a chamfer to make it easier to start the thread.
Here is the proto-nozzle with 3/8" thread.
The tapered reamer I have ordered has a minimum diameter of 3mm so I drilled a 3mm hole through the nozzle.
Here is the partially finished nozzle attached to the 3/8" ball valve.