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Wear of piston ring and cylinder was modelled through a computer code that calculates the hydrodynamic and roughness contact pressures acting on the contact surfaces. Both pressures are fully and coupled solved through, respectively, Reynolds equation and Greenwood-Williamson model. Piston secondary motion and piston groove thermal deformations are considered. The latter was discovered to be fundamental in defining the top ring worn profile. Due to the rough contact pressures, the model predicts material removal from both piston ring and cylinder surfaces and recalculates the system, hence simulating the evolution of the worn sliding surface of both parts. The predicted wear of the piston ring pack and the cylinder wall are compared with a medium duty diesel engine tested for 750 hours in dynamometer.