An assessment model has been developed to determine the piston slap impact forces. This model uses Reynolds' theory for fluid film squeezing to obtain the time history of this impact force. The oil is treated as a damper and the air bubbles entrapped into the oil as a spring. An error of up to one order is found when the stiffness due to the entrapped air is not considered. In this case, the Reynolds theory over predicts the impact forces, compared with experimentally obtained impact forces. The results of this development are used to determine the piston slap in a motored internal combustion engine.