An Otto cycle, Atkinson cycle or supercharged internal combustion engine (10) and a timing mechanism therefor. The engine comprising: a cylinder (14) and a piston (12) reciprocally mounted within the cylinder (14) in which reciprocating movement of the piston is converted into rotational movement of an output shaft (28) by way of rollers (20, 24) engaging primary (22) and secondary (26) cam means which are affixed to, and rotatable with, the output shaft (28). The timing mechanism comprising cam means (52, 53) mounted on, and rotatable in unison with, the output shaft (28), a cam follower (54, 55) arranged to engage the cam means (52) and a linkage connecting the cam follower to a pivoting rocker arm for actuating the engine's induction and exhaust valves, the rocker arm pivot being adapted to be moveable on an arcuate locus centred on the axis of the output shaft (28).