An internal combustion engine enabling asymmetric port timing has an engine mechanism including a crankshaft having a crankpin to which each piston of an opposed pair of pistons is connected and by which the pistons are reciprocatable in a respective one of axially inline cylinders. The mechanism also includes a crosshead having opposite ends on each of which a respective piston is mounted, and a coupling between the crosshead and the crankpin by which the pistons are caused to oscillate circumferentially as the pistons are driven to reciprocate.