A small air-cooled internal combustion engine includes an aluminum cylinder block, an aluminum cylinder head welded to the aluminum cylinder block, and a weld securing the aluminum cylinder block to the aluminum cylinder head, wherein a joint having a first length is formed between the aluminum cylinder block and the aluminum cylinder head and wherein the weld extends for a second length that is at least 25% of the first length.

Provided is a head cover structure of an engine, which is capable of increasing supporting rigidity of a head cover. The engine includes a driving force transmission mechanism for transmitting a driving force of a crankshaft to a valve train, in which the head cover includes a bulging portion for covering an upper side of the driving force transmission mechanism on one end side thereof in a cylinder row direction, and an engine mounting portion is attached to the bulging portion. The bulging portion is provided with an inclined surface portion having an inclined surface inclined downwardly toward the other end side in the cylinder row direction, and the inclined surface is formed to rise upwardly from an intermediate position in the cylinder row direction so as to be continuous with a top surface of the bulging portion on the one end side in the cylinder row direction.

An engine includes: a cylinder block that has a cylinder included in a combustion chamber; a crankshaft that is placed in the cylinder block; a flywheel that is mounted to one end portion of the crankshaft; and a flywheel housing that is placed on a side of one end face of the cylinder block in a direction of the crankshaft and that houses the flywheel. The flywheel housing has a blow-by gas path through which a blow-by gas passes.