A variable displacement piston assembly including a plurality of cylinders, each cylinder having an axis substantially parallel to the axis of the other cylinders, a plurality of pistons, one piston in each cylinder, and each piston and corresponding cylinder having a displacement. The assembly also including a transition arm having a connector pin for each piston, each connector pin connecting each piston to said transition arm, a drive shaft connected to the transition arm and a control shaft rotatably coupled to the drive shaft, the control shaft being movable to adjust the displacement of the piston assembly.