A 3/16″ hole drilled at bottom dead center of a small diesel engine allowed the rings to be observed optically and their surface temperature measured by infra red microdetector. Operating severity was increased by raising i) speed, ii) b.m.e.p., iii) liner temperature. Additives, oil viscosity and prolonged running were studied. It was found scuffing was far more prevalent and ring surface temperature far higher than expected. It seems the engine used scuffing as a means of running-in.