Abstract: Expansible chamber apparatus for use either as an internal combustion engine or as a pump comprising a stationary housing, a cylinder block rotatable within the housing about a central axis, a shaft secured to and rotatable with a cylinder block with the longitudinal center line of the shaft constituting the above-mentioned axis. A stationary gear is carried by the housing and surrounds the shaft. The cylinder block has a plurality of cylinders therein with the longitudinal axes of these cylinders being tangent to a circle surrounding the axis. A piston for each cylinder is reciprocable within its respective cylinder and a connecting rod and an eccentric gear are provided so as to connect each piston to the stationary gear for effecting rotation of the cylinder block and the shaft relative to the housing and for effecting reciprocation of the pistons. Inlets and outlets are provided in the housing for the inlet and outlet of fluid to the cylinders as the cylinder block rotates and as the pistons reciprocate.