A pair of opposed pistons (17) are interconnected by a connecting link assembly (19) defining opposed parallel guide surfaces (24), transverse to the axes of the pistons. A drive block (28) is rotatably mounted on a crankpin (16) of a crankshaft by a bearing shell (71a, 71b) and has parallel bearing plates (31) slidably engaged with the guide surfaces (24). Lubricating oil is supplied to ports (59) in the bearing plates, just before each plate is substantially loaded, from leading and trailing drillings (56L and 56T) in the crankpin (16), ports (76, 77) in the bearing shell and passages (73, 74, 75) in the drive block.