An operator for an overhead door, such as a garage door, to raise and lower the door. The operator comprises a chain drive which is rotated about upper and lower pulleys and powered by a drive motor. Preferably, a pair of spaced apart drive chains are employed. A carriage is secured to the one or a pair of drive chains and is releasably connected to the drive chains by connecting blocks extending outwardly from the carriage. The garage door is thereupon secured to the carriage for raising and lowering movement thereof. Each drive chain includes one double-pitch link for connecting the ends of the chain to form a continuous loop. Each connecting block is provided with a releasable pin which extends into the double-pitch link for releasably securing the carriage to the drive chains. Removal of the pin for each drive chain will allow manual movement of the carriage and hence, the door. This construction precludes the connecting blocks or carriage from being driven to the sprocket where they would attempt to drive around the sprocket in the event that limit switches are not properly set. This construction also allows one to manually disengage the drive chain from the carriage in order to move the carriage and door in the event of a power failure.