This invention relates to a reciprocating internal combustion engine having a cylinder crankcase in which a crankshaft is rotatably supported, to which crankshaft there is flexibly joined at least one connecting rod having a piston guided in a cylinder liner, the crankcase having fittings of an oil circulation system and various attached parts in a number independent of the number of cylinders. The crankcases with various numbers of cylinders can be cast, machined and assembled at the lowest possible cost because groups of fittings and attached parts have spacings to one of the two ends of the various cylinder crankcases (1) that remain constant.