Which component is contained in the head of a jack?

The head of a jack typically contains packing, which serves an essential function in maintaining the hydraulic integrity of the system. Packing is located around the plunger to create a seal that prevents hydraulic fluid from leaking out during the lifting or lowering process. This sealing ability is crucial as it ensures that the jack can hold its position under load without losing hydraulic pressure.

In contrast, the guide bearing is responsible for stabilizing the movement of the plunger but is not typically found in the head of the jack. The plunger is the moving component that extends and retracts to lift the load, while the cylinder contains the hydraulic fluid and provides the necessary force for operation. Both the plunger and the cylinder are integral parts of the jack but are located elsewhere, not specifically housed in the head. Therefore, packing is the correct component associated with the head of a jack.