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Expansion tank

Jaguar Expansion tank
engine cooling system. Stainless steel version of the original tank

Jaguar XK120 (1949-54) • XK140 to May 1955: OTS to 811423 (LHD) and 800036 (RHD), FHC to 814240 (LHD) and 804141 (RHD), DHC to 817459 (LHD) and 807127 (RHD)

Modern cooling systems work mostly with relatively high pressure, so the cooling water temperature can attain a higher temperature to prevent evaporation.
The original solution allowed the precious (see below) cooling water to escape directly through a small hose at the filler neck onto the road. Expansion tanks take the high pressure coolant and pass it back to the system after it has been cooled down thus avoiding the need to keep topping up with fresh water. It is much better to have water without any residual oxidation potential in a long-running system! This modification results in a pressure rise curve which is flat and smooth and the system is not constantly under severe pressure as the expansion volume in the radiator itself is only minimal.