Abstract: A .[.pair of.]. cylindrical .[.rotors are.]. .Iadd.rotor is .Iaddend.mounted for rotation within a stator and .[.each rotor including.]. .Iadd.the rotor includes .Iaddend.a plurality of cylindrical bores annularly around the axis thereof and parallel to the stator bore. Pistons reciprocate within these bores and connecting rods or links of the pistons engage within the profile of a .[.pair of annular cams.]. .Iadd.an annular cam .Iaddend.secured to the stator .[.at opposite ends thereof.]. so that as the .[.rotors rotate,.]. .Iadd.rotor rotates, .Iaddend.the pistons reciprocate due to the cam profile. By providing a set of pistons and cylinders, on each side of a central common cylinder head at each end, a pair of rotors and a pair of cams one at each end of the stator, balance is achieved. A common combustion chamber is provided in the cylinder head for the pistons on the opposite sides of the cylinder head.