A simple method is frequently used to calculate a reciprocating engine’s bearing load from the measured cylinder pressure. However, it has become apparent that engine downsizing and weight reduction cannot be achieved easily if an engine is designed based on the simple method. Because of this, an actual load on a bearing was measured, and the measured load values were compared with a bearing load distribution calculated from cylinder pressure. As a result, it was found that some of actual loads were about half of the calculated ones at certain crank angles. The connecting rod’s elastic deformation was focused on as a factor behind such differences, and the rod’s deformation due to the engine’s explosion load was studied. As a result, it was found that the rod part of the engine’s connecting rod was bent by 0.2 mm and became doglegged. Additional investigation regarding these findings would allow further engine downsizing.In this paper, the authors mainly report that an actual load on a bearing during the engine’s combustion could not be determined by simplified calculation based on the engine’s cylinder pressure, and that it is necessary to take into account a connecting rod’s deformation as well as a piston ring’s friction and a crank shaft’s deformation which were not measured at this time. As a follow up of this report, the authors plan to report minimum oil film thickness and the distribution of oil film thickness with respect to an engine’s individual rotation speed when Elastohydrodynamic Lubrication (EHL) is used.