Abstract: A cylinder block for an internal combustion engine, comprising: (a) a siamesed cylinder sleeve unit (steel or metal matrix composite) having an annular tongue flange spaced above but adjacent the bottom of the unit; and (b) a plastic jacket (phenolic, vinyl ester, or epoxy) spaced from but surrounding the sleeve unit and having the tongue flange molded integrally with the jacket, the sleeve unit and jacket being adapted for closure by a head having a wall to mate with the planar top of the sleeve and jacket; and (c) sealant deposited at interfacing surfaces of the sleeve unit and jacket which are integraly molded, the sleeve being constituted of a metal-based material having a thermal expansion characteristic differing from the plastic jacket by no greater than 25%.