Engine Tests Reveal Effects of Optimized Piston Skirt Thickness: Using a narrower piston skirt is one of the approaches to reduce both weight and friction of a piston. Hitachi engineers measured frictional force of both a stock piston and a prototype piston with the narrower skirt width. The thickness of the piston skirt at both upper and center skirt was varied as a test parameter. The floating liner method was used to measure the piston frictional force in an operating gasoline engine having bore and stroke of 89 and 86 mm, respectively. Both piston frictional force measured per crank angle degree and FMEP were evaluated to characterize the effect of the piston skirt thickness on the piston friction. Some critical information was not revealed in the reference literature, e.g., the precise piston skirt specifications that significantly reduced piston FMEP, probably for proprietary reasons. However, the results presented in the literature are convincing enough to indicate the importance of carefully selecting appropriate piston skirt thickness. This chapter reports the measurement results of piston frictional force, friction power loss, and FMEP obtained through parametric tests and discusses key findings including observations made by iTEP.