For anyone else that might be thinking about doing this, here's my picture guide.
Spare barrel cap, carbonator cap, top cut off a pet bottle (and a hole saw).
The shoulder on the bottle is 33mm diameter and the thread 28mm.
I enlarged the hole to fit the pet bottle top -
The hole needed a bit of filing, but the shoulder on the pet bottle isn't that much bigger than the thread, so a tight fit was essential.
With the carbonator cap screwed fully on, there was still a gap of 4mm -
..so a thick gasket was needed. I used 2 washers cut from a piece of nebar. Nebar is supposed to be a gasket for liquids, but works up to about 75psi so I doubt it will have much bother holding back 5 - 10 psi of CO2 -
Two layers together were just the right thickness.
I cleaned everything up, vaselined the gaskets & o rings, refitted the PRV and fitted the lid to a spare barrel for testing.
And finally pressured up to about 4 psi for leak testing. No discernable leaks after the first hour, but I'll leave it overnight to be sure.
If all holds up, I'll swap the cap over to my barrel full of ale. All in all, a successful afternoon.
