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The two main noise sources of an engine are the combustion process and mechanical impacts. Of the latter, generally the most important is piston slap, which is produced by impacts between the piston and cylinder wall. These impacts cause the engine outer surfaces to vibrate and consequently radiate noise. In this paper the time history of a piston slap impact force model was developed. This model takes into account the influence of the oil film on the impact behaviour, which was found to be an important factor. Test rigs were built to study these effects on the impact phenomenon and extensive tests were carried out.