A rotary actuator (400, 800, 1200, 1500, 1750, 1900, 2200, 2300, 2600, 3200, 3300) comprising: a housing; a first piston housing assembly comprising a first cavity, a first fluid port in fluid communication with the first cavity, and an open end; a rotor assembly rotatably journaled in said housing and comprising a rotary output shaft and a first rotor arm extending radially outward from the rotary output shaft to a first distal end comprising a first bore and one or more first retainers; an arcuate-shaped first piston disposed in said housing and configured to allow reciprocal movement in the first piston housing assembly through the open end and along a curved path of motion having a radius of curvature, wherein a first seal, the first cavity, and the first piston define a first pressure chamber, and a first portion of the first piston connects to the first rotor arm at a first end portion comprising a second bore and one or more second retainers, and a first connecting rod wherein the first piston and the rotary assembly are configured such that rotary movement of the rotor assembly urges movement of the first piston along the curved path of motion having the radius of curvature, outward relative to the first piston housing assembly, and movement of the first piston along the curved path of motion inward relative to the first piston housing assembly urges rotary movement of the rotor assembly, and wherein the first retainers and the second retainers are intermeshed in a direction along the curved path of motion to prevent separation of the first piston from the first rotor arm and the first connecting rod is located within the first bore and the second bore when the first retainers and the second retainers are intermeshed. a housing; a first piston housing assembly comprising a first cavity, a first fluid port in fluid communication with the first cavity, and an open end; a rotor assembly rotatably journaled in said housing and comprising a rotary output shaft and a first rotor arm extending radially outward from the rotary output shaft to a first distal end comprising a first bore and one or more first retainers; an arcuate-shaped first piston disposed in said housing and configured to allow reciprocal movement in the first piston housing assembly through the open end and along a curved path of motion having a radius of curvature, wherein a first seal, the first cavity, and the first piston define a first pressure chamber, and a first portion of the first piston connects to the first rotor arm at a first end portion comprising a second bore and one or more second retainers, and a first connecting rod wherein the first piston and the rotary assembly are configured such that rotary movement of the rotor assembly urges movement of the first piston along the curved path of motion having the radius of curvature, outward relative to the first piston housing assembly, and movement of the first piston along the curved path of motion inward relative to the first piston housing assembly urges rotary movement of the rotor assembly, and wherein the first retainers and the second retainers are intermeshed in a direction along the curved path of motion to prevent separation of the first piston from the first rotor arm and the first connecting rod is located within the first bore and the second bore when the first retainers and the second retainers are intermeshed.