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A theoretical model was developed to better understand the lubrication of piston rings. Models like this are important for studying oil consumption and its contribution to emission. The model suggests that temperature gradients and viscous heating in the oil film can be neglected if it is assumed the oil is the same temperature as the cylinder wall. A simple model suggests that vibrations will not affect the calculated film thickness significantly. Oil film thickness was measured in a Cameron Plint wear test rig and in a diesel engine. Evidence of oil starvation was observed in both tests. Measured and predicted film thickness correlated well on the wear test rig. Engine tests showed some unexpected trends, however the theory predicts oil film thicknesses of the same order of magnitude as the measured results. Measured results showed that separation occurs at the rear boundary of the ring. For modelling, the Reynolds boundary condition has the best correlation with experimental data.