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Groove? Its not any grooves to talk about i belive, if you zooming the first picture on the vertical black line you can see the cross hatching through the black carbon, it looks like you have drag a pencill on the cylinder wall. Possible a sharp edge on a ring that scratch a little that allows a little build up with carbon or something, its also carbon in a little wider span not just in that particular vertical line. I was jus courious what makes it be there.