In addition to what has been written before the essential difference between the two is the suitability for it's end use.

A ringed engine is not suitable for high rpm engines. The ring floats in it's slot. On each stroke of the piston the ring floats up and down, albeit a small amount. On a high rpm engine the ring is battered up and down and very soon goes into self destruct mode.

That is the reason for the ABC type of engine. Nothing to do with preventing damage during lean runs.

Any engine relies on the correct piston/liner seal at TDC. This tight seal can not be for the length of the stroke or the engine would be too tight to turn over, hence the taper. The correct running fits for this type of engine are when the engine is at operating temperature, not when it is cool. The piston/liner materials are such that the liner expands slightly more than the piston at temperature to establish that fit. The worst thing that can happen to this type of engine is to continually turn it over when cold, or, run it too cool.

ED S