Hi Dougiel, my C-21 steam dome does unscrew from it's base (I just undid now) but I seem to remember many years ago when I needed to change the safety valve setting it was near impossible to move. This was probably due to thread grip and the paint 'glueing' it all in place.
I used a piece of thin leather belt to wrap around the middle of the dome and put a long handled adjustable vice grips on this to get good leverage and had someone hold the loco. I got it undone after some huffing and puffing and the extra grip of the leather was a real help and protected the dome from vice grip damage marks. I used various rubber pieces that kept slipping on the dome so no grip when you needed it, old worn leather was best.
The secret is getting some good leverage and some good grip on the dame without squashing it.
Tell your friend I hope he has good luck and to oil the joint and not screw it down very hard for future removal.
Russell

 

Thank you for this information and method you used. We did try some rubber wrapped round a pair of vice grips but were unable, so far, to remove the dome. Have sent this information on to the owner and hopefully he will be able to move it.

DougieL

 

I think there is a thread on this somewhere and it was REALLY hard to unscrew. try searching