The big driver, as I understand it, is not an excess of non-conforming areas between the ring face and the cylinder wall, but rather radial "flutter" of the piston ring under conditions of rapidly changing cylinder pressure. With four bolts per cylinder, the bore distortion will tend to have fourth-order and lower components, to which most compression rings could readily conform. The concern is that cylinder pressure will rise much more rapidly than top groove pressure due to restricted flow between the top land of the piston and top groove. When the pressure acting on the exposed portion of the ring face is sufficiently high compared to the pressure in the groove, the ring can lift off of the cylinder wall and allow large amounts of gas to bypass.
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