Well,
I left a part in some vinegar overnight. Worked a treat. Rub with a toothbrush, rust gone!

Robinson Hot Air Engine.Today was shopping day. At 13p a bottle, I stocked up..... Other parts are pickling now.

Robinson Hot Air Engine.Dean, Nick. Not sure I understand your gasket theories. Sorry!

The port tube (if it is a through tube), is threaded at 90*, through the joint, but offset by some 75%.....
Robinson Hot Air Engine.Sorry, the female thread is out of focus!

Robinson Hot Air Engine.I think this is the other end of the tube, and should have come out first. But, it won't unscrew, and the brass plug was a press (hammer?) fit.
Guess it will need drilling out.....
Robinson Hot Air Engine.Things should be clearer tomorrow, when the vinegar has done it's job.
David D