A small air-cooled internal combustion engine includes an aluminum cylinder block and an aluminum cylinder head welded to the aluminum cylinder block.

A composite cylinder block for an internal combustion engine includes a main block body made of a metal material and an outer member made of a synthetic resin material. The main block body has a lower deck, a base part and a cylinder wall. The outer member has an upper deck and a water jacket constituting wall surrounding the cylinder wall to define a water jacket. An outer-member-side mating surface is provided with a welding rib on a lower end of the water jacket constituting wall and is heat-welded to a top surface of the base part by heating the main block body from the lower side and pressing the outer-member-side mating surface against the main block body. The welded surfaces of the main block body and the outer member are in one plane orthogonal to a cylinder axis direction.

A composite cylinder block for an internal combustion engine includes a main block body made of a metal material and an outer member made of a synthetic resin material. The main block body has a lower deck, a base part and a cylinder wall. The outer member has an upper deck and a water jacket constituting wall surrounding the cylinder wall to define a water jacket. An outer-member-side mating surface is provided with a welding rib on a lower end of the water jacket constituting wall and is heat-welded to a top surface of the base part by heating the main block body from the lower side and pressing the outer-member-side mating surface against the main block body. The welded surfaces of the main block body and the outer member are in one plane orthogonal to a cylinder axis direction.