A single cylinder, internal combustion engine with a dry sump lubrication system. The engine includes an engine housing in which the overhead camshaft and crankshaft are rotatably supported, and the housing includes an integrally formed cylinder and head. A timing belt disposed externally of the engine housing interconnects the crankshaft and camshaft, and a piston connected to the crankshaft reciprocates within an internal bore provided in the engine housing cylinder. The cylinder wall around the internal bore is of a generally uniform thickness and circumscribed by cooling fins such that the cylinder resists bore distortion during operation. Dry sump lubrication is obtained by an external oil reservoir connected to a pump which supplies pressurized oil to the bearing journals of the camshaft. A portion of the oil at the camshaft bearing journals flows through passages provided within the cylinder to lubricate the bearing journals of the crankshaft. The reciprocating motion of the valve assemblies controlling intake and exhaust of the combustion chamber pumps the oil which lubricated the camshaft back to the external reservoir. The reciprocating motion of the piston similarly effects a high pressure within the crankcase cavity to pump oil which has lubricated the crankshaft back to the external reservoir. The inventive engine further provides for the mounting of flywheels within the crankcase cavity in conjunction with an external, lightweight fan for engine housing cooling, as well as employs a cast in valve seat for the overhead valve assemblies.