It's only increasing the sides of the diameter, not the whole circle inside the valve. Nothing to do with depth, just width at the areas where the oil flows out as indicated by his yellow lines. Once you see the part, you'll understand it's a small amount of opening it up, probably less than 2mm each sideFor clarification your saying don’t make the ID too big? Or are you saying don’t go too deep? I can’t tell from the photos if the entire ring is thinned from the inside out or if it’s only about half way down. If I held the part I’m sure it would make more sense, but can any describe the process with some crude measurements perhaps?