Header tank cap vs expansion tank cap

Hello all,

I got all all mixed up re which one needs a sprung pressure cap: the header tank or the expansion tank (my 1975 XJ6 is stock and does not have an overflow tank).

I reason that the expansion tank needs a sprung cap as it is a mere “extension” of the header tank, and coolant movement to it shouikld allowed freely but… looking for info on the web is showing everything and its contrary while the terminology is very … variable.

My understanding is the following:

  1. Header tank: a small chamber at the front of the manifold. Its purpose is to increase the volume of coolant available to the engine and allow the filling up of the system.
  2. Expansion tank: a ~2 liter tank placed on the inside of the front left fender that serves to accommodate the volume fluctuations of the coolant as the engine temperature goes up and down. It should never be empty to avoid air going into the cooling system. If it is overfilled, when the engine is warm, excess coolant would be pushed out of the system into the outside world (or an overflow tank if fitted).
  3. Overflow tank: to catch possible coolant overflow and avoid spillage: whatever gets in there, never goes back into the cooling system, it is waste.

Please put me out of my misery, I will take the humiliation :slight_smile:


The pressure cap should be on the 2 litre tank and the plain cap on the engine.


A proper overflow tank (as in later cars) catches the excess; when the coolant cools back down the excess gets sucked back into the expansion tank (instead of air) via a valve in the cap. It’s better than just a catch-the-spillage-bottle in that way. Although any bottle should work that way if the hose is shoved in deep enough. The plain cap goes onto the thermostat housing as he said.


The plain cap goes on the RADIATOR. There is no cap on the engine or the thermostat housing of an E-Type.

A header tank is an expansion tank. On many US cars, the top tank of the radiator serves this purpose. OTH, an expansion tank isn’t necessarily a header tank. The S2 tank serves the function of an expansion tank, but the system can’t be filled through the tank. That’s why you need two caps.

A recovery tank is a non-pressurized tank that captures overflowing coolant and can return it to the radiator when the engine cools. There is no header or expansion tank when a properly designed recovery system is used, but the cap has to be a vented cap (e-type caps are non-vented).

1 Like

1976 S2 XJ!
Wrong list. Robin and I know what the E Type is like and what the S2 XJ should be like. The thermostat housing is basically the radiator, there’s a rubber hose in between, and the additional vent/ expansion hose, that’s all.

1 Like