Measurement of piston frictional forces during engine operation is valuable for improved fuel economy engine design. The measurement, however, is not easy work because the frictional forces are small compared with the gas and inertia forces. Several years ago, at the Musashi Institute of Technology, frictional forces were measured with a movable bore with pressure balancing. Recently, the pressure balancing devices have been improved and adapted for small engines.The piston frictional forces in a small diesel engine and a gasoline engine have been measured with the new device. The characteristics of the friction forces and the comparison between engine sizes, gasoline and diesel engines have been clarified and the effect of multi-grade oil and friction modifiers have been tested.