Pinch :When the sleeve is manufactured it has a slight taper from the bottom(largest) of the sleeve to the top(smallest. The reason for the taper is that as the engine heats-up, the top will get hotter than the bottom of the sleeve and will expand more than the bottom, so when the the engine is hot the sleeve has the same diameter from top to bottom. The same principle is used in airplane engines(real airplanes, piston powered).