OK so I'm up for a bit of experimenting here.. I bought a couple of 22mm id 3mm cs O-rings (shore70) to see how they would fit.. Easy on to the piston and slides nicely
into the bore with some but not a huge amount of resistance - feels pretty good. I have some concerns about the 'side slop' in the 3.5mm groove but the proof of the pudding and all
that. Hope I can the old girl back together tonight for a test out.

Cheers

James