Abstract: A declutchable manual override gearbox, comprising a casing, and a drive shaft mounted for rotation within the casing. The drive shaft has an input end and an output end, the input end of the drive shaft adapted to engage the output of a rotary actuator and the output end of the drive shaft adapted for attachment to a device or mechanism, for example the drive linkage for a damper valve, to be driven by the actuator. A manual override is provided for manually rotating the drive shaft, the gearbox also including a clutch arrangment for selectively rendering the manual override inoperative. Attachments associated with a base portion of the casing enable mounting of the gearbox on an underlying support stand.