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Diesel engines make a shrill noise called “idle knock” under low temperature idling operation. This causes a serious noise pollution problem in automobile diesel engines. It was clarified by this study that one important source of this noise was piston slap impulse. Piston slap motion was measured under usual operating conditions and a condition with additional oil supplied into the piston clearance. The piston slap motion was calculated taking into account the frictional resistances of the crank mechanism and squeeze action of oil film. It was concluded that only a negligible amount of oil existed in the piston clearance for the squeeze action.