I was thinking about my terminology "ring groove" this weekend, before I saw your questions about it. My terminology may have been off. What I was referring to is the piston ridge (not even sure of that term) at the top of the cylinder..... from wear. It is far greater on this cylinder than any of the others. I haven't measured it because of lack of a decent set of calipers. I have pulled the heads off. This cylinder is also much more sootier and dirtier than the others. I am also basing my statement that it is a stock bore because of the pistons having no .010, .020, etc stamped in the face.