Abstract: An expansible chamber device includes a rotating piston braking system and a rotating piston synchronizing system. The rotating piston braking system controls the motion of the expansible chamber device piston assemblies to cause intermittent rotation of the piston assemblies in the same direction during recurrent periods of rotation, with each of the piston assemblies being stopped between the periods of rotation. The braking system includes a set of cam surfaces on the piston assemblies and a set of movable members adapted to alternately engage the first and second set of cam surfaces to stop the rotation of first piston assembly while permitting second piston assembly to rotate freely. A pair of elongate pivotable members engage the piston assemblies on one end and engage a slidable member on the other end. The slidable member and the pivotable members alternate between first and second positions in response to engagement with ramp and stop surfaces provided on the piston assemblies.