coefficients of expansion ...
because of their annular construction, rings don't *effectively have a cooefficient of radial expansion. As they heat up, the expansion would be longitudinal-circumferential, around the ring, and not transverse, like across the top-diameter of a piston as it changes temp. The piston would change diameter, as would the cylinder walls, which being a different material, would probably expand at a different rate, have a different coefficient of expansion than the piston. If a ring (or rings) is the only thing providing the seal between these two changing elements, then, up to a point, wouldn't a thicker ring system make a better seal? (On the other foot, it might also be that the thicker the ring, the longer it would take for it to "find its seat" during break-in (there being nothing manufactured perfectly round in the universe). This would mean that a system with 2 thin rings might have a faster break-in period than a system with 1 thick one, though both would provide the same resistance to pressure bypass.)