This work presents the results of the study of the forces involved in the rubbing friction between the moving parts of the Otto cycle internal combustion engine. In order to study the friction force, a Honda GX 35 engine was modified and a load cell was attached to its chassis. The friction forces among the internal parts of the engine were transferred to the engine chassis, and, by means of a support, to the load cell. Those forces were measured in several situations of the engine, making possible to identify the amount of friction related to each component. The total measured friction power was equal to 112W, representing about 10% of the developed power of the engine. The results obtained by the tests showed the contribution of each individual part of the engine on the friction losses. By the results, it was possible to propose modifications to reduce the total internal friction of the engine, in order to increase its efficiency. After the measurements of the friction force related to different components of the engine, new measurements were carried out in order to analyze the influence of the geometry of the pistons on the friction. During the tests, they were employed pistons with different geometries in relation to the original one. When a modified piston was employed, a reduction of up to 24% could be obtained on the values of friction when compared to those ones produced by original crankshaft - rod - piston assembly. Most of the friction energy is dissipated as heat through the coolant and lubricant. This heat is removed from the system by the water and oil radiators, so the friction losses also have great influence in the cooling system design.