Method for forming a gearbox casing for a plurality of engines for motor vehicles, and casing produced by this method

A gearbox casing (21) for a plurality of engines for motor vehicles has a housing portion (22) for accommodating the gearbox and an attachment portion (23) for connecting the housing portion (22) to one of the engines; the housing portion (2, 22) is made so that it is common to all the engines, while the attachment portion (23) forms a separate adapter element which is interposed between the housing portion (22) and the corresponding engine and which is made in a dedicated mould according to the specific identified characteristics of the engine to which it is connected.