The employee was replacing an O-ring that sits on top of a hydraulic oil accumulator tank. The O-ring seals the accumulator tank and the oil manifold to the central cylinder, which had been removed for repair. The employee used a hydraulic jack to raise the oil manifold approximately 1 inch to allow for installation of the O-rings and the central cylinder. The jack slid from its position and the manifold dropped back to its normal resting position, striking and pinching the employee's right thumb. The employee sustained a fracture and open laceration to the thumb, near the knuckle joint.