PURPOSE: To maintain substantial strength and to make as small as possible the section of water paths by casting pipes for the water paths between cylinder bores and opening both ends thereof to the surface side to be connected to a cylinder head.

CONSTITUTION: The water paths consisting of the pipes 7 which are formed partly flat are cast between the cylinder bores 2, 3 and 3, 4 in the stage of forming the cylinder block. Both ends of the pipes 7 are bent to a U shape and are directed through water jackets 5, 6 to the surface of the cylinder block 1 to be connected to the cylinder head and are opened on said surface.