WATER JACKETS
Water jackets are channels that line each cylinder; they are cast into the engine
block and cylinder head(s) when they are made. Engine coolant flows through
these channels, absorbing the heat of the combustion chambers (cylinders). Keep
fresh coolant in your engine to keep these channels clean and free of rust and
scale. The drain and refill interval should be two years or 25,000 miles, unless
otherwise stated by the carmaker.