Well now I have to say I know of one conventional 4 stroke that is ringless, the OS 70FL. I see in the graphic that they connect the crankcase to the muffler. I guess that's how they oil the bottom end. I doubt it's to vent the case to the exhaust. On a ringless ABC I would doubt the blow by pressure would be enough to win over exhaust pressure. I'm not sure I like the idea of oil out of the muffler for lube. If they are still relying on blow by to lube the bottom that would take a pretty loose fit piston even considering the two oil groves. It'll be interesting to see how these work out.

Denis