A theoretical model has been developed which simulates piston movement in the engine cylinder. Piston movement is calculated by a precise step-by-step method in order to accurately obtain the intensity of piston-bore impact. The intensity is defined by both impact force and energy.It is shown that this model gives good agreement with measured piston movement. The impact intensity predicted by this model is confirmed by measuring the cylinder wall vibration.Piston side force obtained by this model gives a good prediction of piston-skirt wear. An example which shows the reduction in oil consumption improvement of the piston movement is also demonstrated.