I've only ever come across one on a MK4 Airsporter and it went straight in the bin, as I do with all BSA Nylon piston heads.

From memory the O ring groove is over the cross pin so it can't escape, and the steel ring seemed to be there to centralise the piston head in the cylinder bore.

The type of Nylon the BSA piston heads were made from takes in oil and swells over time sapping power, so I always fit an aluminium piston head to avoid this problem.



All the best Mick