Most lifters are convex on the bottom- - - -slightly higher in the center than at the edges. As the cam lobe turns under the base of the lifter, the radiused bottom causes the lifter to spin, preventing a straight across wear pattern from developing on the bottom. Some engines also offset the cam lobe a few thousandths away from the center of the lifter to produce a rotation as well as running a radiused lifter bottom. The radius is usually very slight, like the perimeter of a 10 foot circle- - - -about .001" height difference from the center to the edge of the lifter bottom- - - -less than the thickness of a strand of hair.
Jerry
