An internal combustion engine having a rotor member journaled in a casing member for rotation about a first axis. A cylinder is provided on one of the members and has a piston received therein which oscilates between extended and retracted positions. The other of the members is provided with a cam surface having high and low extremities and there are means on the piston forcibly engaging the cam surface, the engaging means in the retracted and extended position engaging the high and low cam surface extremities respectively. The engaging means reacts against the cam surface along a force line laterally off-set from the axis of the rotor member and at generally right angles to a plane therethrough, whereby extension of the piston from its retracted to its extended position causes relative rotation of the rotor member with the engaging means moving over the cam surface from its high towards its low extremity and its point of engagement therewith at all rotational positions between the extremities of the cam surface being on the aforementioned force line.