Thanks. I can at confirm hat the BS212 (21.82 ID 28.88 OD is too tight at least in combination with Chamber's alloy piston as you push it through the threaded section it bucks up at the slotso I cut it in the end to get it off. My digital verniers are out of commission at present but the old verniers measure an ID of 22.2 ish on the piston ring slot and about 27.4mm on the outside of the piston. With this in mind I guess we should be aiming for a BS 211 (20.22 ID 27.28 OD) or metric might be better at (21 ID 28 OD). @ CapitalB ee do you know what your size is? These things are easy and cheap to source on - once you know what works!