I understand what you are saying. But don't forget the ring doesn't always fill the entire ring groove. There is clearance behind the ring....... between the inside of the ring groove and the ring. The gap still needs to be large enough to prevent the ends of the ring from touching when the engine is at max operating temp.

Yes, the aluminum piston and cylinder will expand more than the iron ring. And you would think that would provide all the clearance needed to prevent the ring from binding up. But you have to consider the ring IS going to expand. Not as much as the aluminum, but it WILL expand. As the metal expands, the ends of the ring will get closer together. The ring gap will decrease with heat. Thats why we need more than .002" . It's not because the ring will expand out and hit the aluminum. It's because the ring will expand and the gap will close up. Once the gap closes up completely, the ring will start to get oval instead of round, as it continues to expand. [X(] Then it goes outward and hits the aluminum cylinder and creates more drag/friction in a few hot spots instead of even pressure on the cylinder walls all the way around the ring.