Air leaking past piston rings?
The problem is that the pressure is not evenly distributed all the way around when you put the ring in a smaller bore.
I honestly don't think this is the case. Draw the ring and put force arrows all around it at 1 degree intervals (a free body diagram) to indicate forces on the ring. The forces are constant all around. There is nothing there to bend the ring differently in 1 place versus another. The end gap is the only discontinuity, and that exists on all rings and is about 0.1% of the total circumference.
I believe you are thinking of what happens if you apply the force only at the ring ends; in that case it will indeed distort the ring from a nice circle.
Now it may be that, with enough side pressure, the ring will eventually tend to buckle a bit and get 'wavy' as viewed from the side. In that, case, I am 100% with you in expecting variable ring seal.
If this was the case they would just make one oversize ring for every application and then just file fit them all.
I don't think so; I thought the rings'
curvature is varied to give the right wall pressure. But see below.
When the ring OD gets bigger, so does the ID. You can file the ends down all you want, the ID will still be wrong.
But if the rings width is the same for all oversizes (which you seem to be saying) and the OD is squeezed down, then you would end up with the same ID, right?
I thought they just built them with different curvature and kept the same exact width for all bore sizes, but realize now that I really don't know that answer.
