A pneumatic valve actuator for moving a valve stem of a control valve, the actuator comprising: a diaphragm casing comprising an upper casing and a lower casing; the upper casing having an upper flange and the lower casing having a lower flange; a diaphragm plate movably disposed between the upper casing and the lower casing, the diaphragm plate arranged to be operatively coupled to the valve stem to move the valve stem; a spring disposed between the upper casing and the lower casing and arranged to bias the diaphragm plate in a first direction; a diaphragm disposed between the upper casing and the lower casing, the diaphragm including a central portion movable in conjunction with movement of the diaphragm plate; the diaphragm including an outer portion sized to fit between the upper flange and the lower flange; an air inlet defined in the diaphragm casing and arranged for attachment to an air supply source, the air inlet permitting the air supply source to bias the diaphragm plate in a second direction; a retention ring disposed between the upper flange and the lower flange and positioned to engage the outer portion of the diaphragm; wherein the upper flange and the lower flange have attachment holes (88, 90) each sized to receive a fastener, wherein the retention ring includes an outer portion having holes alignable with the attachment holes of the upper flange and the lower flange, and wherein the diaphragm includes attachment holes in the outer portion alignable with the attachment holes of the upper flange and the lower flange; characterized in that: the retention ring includes a protrusion (96, 178) positioned to engage a surface of the outer portion of the diaphragm, and in that a plurality of inserts are disposed between the upper flange and the lower flange and positioned within the attachment holes of the diaphragm, the inserts being sized to receive the fastener. a diaphragm casing comprising an upper casing and a lower casing; the upper casing having an upper flange and the lower casing having a lower flange; a diaphragm plate movably disposed between the upper casing and the lower casing, the diaphragm plate arranged to be operatively coupled to the valve stem to move the valve stem; a spring disposed between the upper casing and the lower casing and arranged to bias the diaphragm plate in a first direction; a diaphragm disposed between the upper casing and the lower casing, the diaphragm including a central portion movable in conjunction with movement of the diaphragm plate; the diaphragm including an outer portion sized to fit between the upper flange and the lower flange; an air inlet defined in the diaphragm casing and arranged for attachment to an air supply source, the air inlet permitting the air supply source to bias the diaphragm plate in a second direction; a retention ring disposed between the upper flange and the lower flange and positioned to engage the outer portion of the diaphragm; wherein the upper flange and the lower flange have attachment holes (88, 90) each sized to receive a fastener, wherein the retention ring includes an outer portion having holes alignable with the attachment holes of the upper flange and the lower flange, and wherein the diaphragm includes attachment holes in the outer portion alignable with the attachment holes of the upper flange and the lower flange; characterized in that: the retention ring includes a protrusion (96, 178) positioned to engage a surface of the outer portion of the diaphragm, and in that a plurality of inserts are disposed between the upper flange and the lower flange and positioned within the attachment holes of the diaphragm, the inserts being sized to receive the fastener. the retention ring includes a protrusion (96, 178) positioned to engage a surface of the outer portion of the diaphragm, and in that a plurality of inserts are disposed between the upper flange and the lower flange and positioned within the attachment holes of the diaphragm, the inserts being sized to receive the fastener.