What is used in the rod side of a cylinder to stop the cylinder rod before it fully extends?

The correct choice is the stop tube, which is a component specifically designed to limit the extension of the cylinder rod. It acts as a physical barrier that engages with the rod when it reaches a predetermined extension limit. This ensures that the rod does not extend fully beyond its intended stroke, which could potentially damage the cylinder or other connected components.

The presence of the stop tube is crucial in applications where precise control of the rod's position is necessary. It provides an effective means to enhance safety and prevent mechanical failure due to overextension.

In contrast, while end caps are essential for sealing the cylinder and providing structural integrity, they do not serve the purpose of stopping rod extension. Similarly, retaining rings are typically used to hold components in place within a cylinder but do not prevent the rod from extending. Locking mechanisms may also involve some interaction with rod positioning, but they usually serve a different function, such as maintaining a position rather than stopping extension. Thus, the stop tube is specifically designed for that crucial stopping functionality in the rod side of a cylinder.