Abstract: A fluid-operated internal combustion engine cylinder cutout device selectively operable to function as a rigid extension of a valve pushrod, as a deactivating means between the pushrod and its respective valve, or as a fluid jack means for holding the valve associated with a pushrod open while the operation of the pushrod is unimpeded. The device enables normal operation of an engine cylinder intake or exhaust valve by its pushrod; selective deactivation of the intake valve operation by uncoupling the pushrod and its respective valve; and sustained opening of the exhaust valve, the valves being associated with an engine cylinder desired to be cut out from engine operation. Engine oil pressure may be used as the operating fluid of the cutout device, which is constituted of an outer cylinder; an inner hollow piston slidable in the cylinder and having a pair of axially aligned, fluid isolated chambers; and a small piston in one of the chambers.