Actually the lifter bottoms are convex, they bow out. you can rock to new lifters face to face and see light between the edges. The lifters rotate because the cam lobes are crowned on the lobe tip, higher on one side than the other so the cam actually only hits the lifter on one side causing it to rotate and that lobe angle should be close to the convex face angle of the lifter. You can see this pattern on a camshaft lobe as one side is worn more. Its been said that you should watch the lifters to see if they rotate on engine break in. put a dab of white on the edge. If they dont then you need to determine why ASAP.
