An employee was repairing a seal on a valve operator. He placed the drum cover over the actuator piston using an overhead crane and was manipulating the drum while operating the crane controls. While attempting to lower the drum the final 2 inches, he inadvertently pushed the wrong button on the hoist control, causing the drum to move upward. As a result, his left thumb was pinched between the base plate and drum cover. There was no amputation.