A new piston secondary motion analysis has been developed that accurately predicts piston strength and the slap noise that occurs when the engine is running. For this secondary motion analysis, flexible bodies are used for the models of the piston, cylinder and cylinder head. This makes it possible to quantify the deformations and secondary motion occurring in each area of the engine. The method is a coupled analysis of the structure analysis and the multi body dynamics analysis. The accuracy of the results obtained in the new analysis method was verified by comparing them to measurement data of piston skirt stress and piston secondary motion taken during firing. To measure piston skirt stress, a newly developed battery-powered telemetric measurement system was used. The calculation results were close to the measurement results both for stress and for secondary motion from low to high engine speed. The same comparison was carried out on the vibrational acceleration produced on the cylinder, to determine how piston pin offset and piston skirt rigidity affect piston slap noise. Those calculation results of the vibrational acceleration also closely matched the results of the measurement. With those results, it has been confirmed that this method is an effective tool to make a comparison among different specifications of pistons. This developed method makes it possible to examine the specifications of pistons which are light weighted but still durable with minimal slap-noise.