It is not the unworn previous bore that constitutes the problem. As the rings and bore wear, they create a very slight rounded edge where the ring stops at TDC and the rings develop a matching slightly rounded edge. Also the ring groove and rings wear letting the the top of the ring be slightly lower. Now if you do not remove the ridge the new square ring that is a thousandth or two thicker will hit the bottom of the ridge with less than beneficial consequences.
