Abstract: This invention relates to a valve gated injection molding system of the type where the melt passage enters the system on general alignment with the valve pin and cavity. The system has a novel valve pin actuating mechanism which is pneumatically operated according to a predetermined timed cycle. A piston which reciprocates in a cylinder has an outer collar portion from which a tongue portion extends inwardly to operatively engage the driven end of the valve pin. The melt passage extends through a portion of the back plate which, in turn, extends through the outer collar portion of the piston and is offset to leave room for the tongue portion of the piston to engage the driven end of the valve pin. Piston rings are located between the piston, back plate and cylinder to prevent excessive air leakage. This center entry system with a pneumatic actuating mechanism provides a more reliable mechanism which may be double acting.