An engine block assembly includes a head structure assembly, and a cylinder structure assembly. A plurality of fasteners interconnect the head structure assembly and the cylinder structure assembly. An outer shell encapsulates and supports both the head structure assembly and the cylinder structure assembly. The outer shell is a unitary polymer structure that is simultaneously over-molded onto thehead structure assembly and the cylinder structure assembly. The head structure assembly and the cylinder structure assembly are constructed of components that are designed to carry the operational loads of the engine with minimal weight. The outer shell provides additional structural integrity, and provides fluid passages for lubrication and cooling, as well as support for other engine components.