Jo,
The standard K & B engines of the 70.s are ringed engines. Most thunder Tiger engines are ringed. The ABC ,is a good engine ,but when its wore out,you have to replace piston & cylinder.
The ringed engine just requires honing the Cyl,and repacing the ring. All the O.S. FX series are ringed engines. The O.S. LA series are cheaper ABC types. My general thoughts have been --the ABC are throw away motors. Run them till they start loosing power ,then buy another one. You can buy two O.S.46 LA motors for the price of one O.S.46 AX.