Brass fire hose and hydrant caps provide good strength, ductility, and corrosion resistance. They are more heat-resistant than aluminum fire hose and hydrant caps. They twist onto fire hydrant and hose openings to keep out dirt and prevent damage to external threads.Fire hose and hydrant caps and plugs are accessories that protect the open ends of hydrants and hoses when not in use. Caps cover fire hydrant and hose openings to keep out dirt and prevent damage to external threads. Plugs screw into hydrant or hose openings to cover and protect both internal and external threads from dirt and damage. Some caps and plugs come with a chain that connects the fitting to the hydrant or hose to prevent it from getting lost.