Does this look like your engine?
http://www.webra-austria.at/motor.ph...r=AERO&lang=EN
This is an ABN engine without a ring. You should have a fairly tight/snug fit between the piston and sleeve near the top of the stroke. If not, it is possible that it is just worn out and needs parts.
Looking into the exhaust port, can you see a ring on the top of the piston?