In an engine of the type having opposite side wall portions more or less differing from each other in rigidity, as in a so-called side valve engine, the axis of the piston unit is set at such an angle of inclination relative to the axis of the associated cylinder bore that the two axes substantially coincide with each other in the power or explosion stroke of the engine cycle when the cylinder block is tilted, under the tensile load acting on the engine structure, on account of the differential rigidity of the opposite side walls thereof. With this axis arrangement, an uneven wear of the mating bearing surfaces of the piston and cylinder and of the crankpin and bore in the large end of the associated connected rod is effectively prevented.