To all those people still struggling to understand the scientific principles, I apologise, unreservedly for starting this thread with photos of work which had no connection whatsoever with injectors.
Blame Neil. It's all his fault really.

Just my implausible attempt at humour, that's all Ian.

I've looked at Gary's link, and examples of injectors – I've made a sketch [I'll apply some rational dimensions to it] I've found a billet of aluminium which will be suitable, and I intend to have a go at making one. It may never work, for no other reason than that I don't have an application for it – but I would like to have a go at it anyway.
I'll post the results, in due course.

I've started on making an injector ~ again, out of a billet of aluminium, for no other reason than this is what I have a stock of, in appropriate sizes. I started by doing a sketch from the *LINK* information which several of you kindly provided; to ensure that I had a steam inlet, a water inlet, an overflow & a safety valve which incorporates the outlet to the boiler. That done I put some dimensions to it, which would enable me to access all of the internal parts to fit the pressure cone. (one cone will do for the purpose of the exercise)
So – the body is machined ~ I don't yet have a mill, so some of the drilling was done in the pillar drill and some done on the lathe in the 4 jaw chuck. All of the openings line up and are where I anticipated they would be [internally] and I have bored out the two ends to take the cover caps which I have also made. These will bolt onto two BA studs each. (I do appreciate that the real injector would have more than two fixings, but two on each end is sufficient for this exercise purpose)
My next task is to machine the four spigots for steam in, water in, overflow and high pressurized steam out. I intend to make the cone and the external connections with brass and use 1/4" copper pipework.

Can you please advise on the thread type and pitch for gland and connection nuts ?

Thanks
Jack