Abstract: In a casting core for the water jacket of a cylinder block in a multicylinder, reciprocating-piston internal combustion engine, whose cylinders are arranged in line and are cast in one piece linked by cylinder webs, these webs are provided with rectangular plates of a ceramics material--preferably sintered oxide ceramics--, which are used as supports for two core halves, in which they are anchored in such a way as to present a one-piece casting core. The supports are removed during the core cleaning process, leaving passages for the cooling water.