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Anti-scuffing performance of Marine Cylinder Lubricants (MCL) is one of the most important parameters for large bore cross-head diesel engines. The causes of liner scuffing in marine crosshead diesel engines have been discussed for some time. It is believed that piston detergency plays a significant role in preventing scuffing because hard piston deposits can initiate the process of liner scuffing. In this study the authors focused on hardness of piston deposits and their composition, because it is believed that excessive liner wear may be caused by harder piston deposits accelerating liner polishing, increasing blow-by and leading ultimately to liner scuffing. In this study several piston deposits from field vessels were collected and the composition of the piston deposits was investigated by thermo-gravimetric analysis and x-ray diffraction analysis. The hardness of actual piston deposits was measured using equipment commonly used in the plastics industry. The authors confirmed that harder piston deposits were present in engines in which scuffing occurred. Following the piston deposit analysis, the authors generated calcium sulfate in the laboratory from commercial MCL's by neutralizing with sulfuric acid at various temperatures from 200 deg C to 400 deg C, and measured the quantity and the hardness of the calcium sulfate produced. The authors also examined the effect of water on the formation of the calcium sulfate. This paper discusses the relationship between the hardness of the calcium sulfate generated in the laboratory and engine scuffing in the field.