Coolant expansion tank

A coolant expansion tank is a small header vessel that absorbs the thermal expansion of jacket cooling water in a closed engine cooling circuit and keeps the system topped up and vented. It sits above the highest point of the circuit so its static head holds positive pressure at the pump suction, preventing cavitation, and provides the surface where dissolved air and combustion gases separate out. The tank carries the make-up connection, a low-level alarm, and a sight glass, and its volume covers the difference between cold-fill and hot-running water inventory in the high-temperature jacket loop.