Cylinder Ridge
A lip of un-worn metal formed at the top of a cylinder bore where the piston rings do not travel.
A cylinder ridge is a raised ring or lip of metal that forms at the very top of an engine's cylinder bore over time. As the engine runs, the piston rings move up and down, wearing away the cylinder walls. However, the top piston ring stops slightly below the deck surface of the engine block. The area above this limit experiences no ring friction, creating a ridge of original, un-worn metal coated with carbon buildup just above the ring travel limit.
The height and depth of the cylinder ridge are indicators of engine wear and mileage. If a mechanic attempts to push the pistons up and out of the cylinders during an engine teardown, the top piston ring will catch on this ridge. Forcing the piston past the ridge without removing it can break the piston land or shatter the piston rings, damaging the piston assembly and potentially scoring the cylinder.
To safely remove pistons, mechanics use a specialized tool called a ridge reamer. The ridge reamer is inserted into the cylinder and rotated, carefully scraping or cutting away the metal ridge until the cylinder wall is completely flat and flush. Once the ridge is removed, the piston and rod assemblies can slide out smoothly without catching.