Ring grooves are machined to specific tolerances, creating a three-sided machined surface. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. Ring lands also function as the running surface for the side of the corresponding piston ring.
The clearance between the piston ring and the ring lands is critical for proper rotation, flexing, sealing of piston rings, and routing of combustion gases. The depth of the ring groove must provide sufficient clearance for the piston ring to compress into the ring groove without contacting the piston.