A barrel internal combustion engine includes a drive shaft assembly, a monoblock, a plurality of piston assemblies, and a valvetrain. The driveshaft assembly has a central drive shaft and a cam plate extending therefrom. The one piece monoblock has a longitudinal central axis with a central longitudinal opening receiving the drive shaft, the one piece monoblock further defines a plurality of combustion chambers, a coolant system, a plurality of intake passages and a plurality of exhaust passages. The combustion chambers each have an axis parallel to the central axis and are defined in a circle concentric with the longitudinal axis. Each chamber has a generally cylindrical side wall, a first closed end and a second open end, an intake valve opening and an exhaust valve opening being defined in the first closed end of each combustion chamber. The coolant system includes a plurality of coolant passages.