Early on the two caps were different but by '89 they were both the same part number CCC6707 @15lbs.

 

 

Look at the rubber seals on both caps. No cracks can be in the rubber or the system will not pressurize.

 

The caps should have a seal immediately under the cap in addition to the seal at the end of the pressure relief spring.

 

 

Confused by your use of both "header"tank and "expansion" tank, l consider them the same thing.
You can get away with no spring seal on the crossover pipe filler as very early V12's did but not on the expansion tank.
Apart from those early motors BOTH caps should be the SAME. If you use a non spring cap on the expansion tank the system will be open to the atmospheric tank and will not hold pressure.

 

Interesting - in the owner's handbook for my 1992 V12, it says very clearly NOT to mix the two caps as they have different ratings. But looking online they seem to have the same part number. Curious.

 

The centre spout cap actually only needs a solid cap at the top, although a sprung cap is fitted OEM, it does not need one. This is because when the pressure spring gives, the overflow connects back to the pressured system via the radtop bleed system, it does NOT vent to an atmospheric bottle.
Whisper it quietly, but it seems Jaguar did not fully grasp this fact when they gave the system two separate pressure caps. The Great Palm explains this clearly.
By contrast, the wing header tank has and must have, a sprung pressure cap, as when that vents it releases to the atmospheric bottle. For years, until I modified my system with a header tank mounted higher than the engine and the rad, and did away with the spouted crosspipe in favour of one from the X300 bodied, V12-engined saloon which has no spout, I used an solid cap on the crosspipe.

 

Where is this atmospheric bottle? I've heard this mentioned before but never found it on my car?

 

It is located in the space behind the front L/H wheel behind the splash guard.
Mine was thrown many years ago. There is no need for it or the rust it promotes in that area of the sill.

 

My car has a rust crack in that exact spot. Now I have my Saturday project. You can even see the bondo patch in my signature picture where it rusted. What did you do with the hose running to that area? I always assumed it was some pressure valve that released to the ground at the end not a god damned metal tank...

 

The hose (on my car) from expansion tank cap spigot runs straight downward and ends below chassis rail. The only time there is any spill from that hose is first time engine warms after coolant refill. The thermal expansion will dump about a half litre from fully filled system that one time only.
The atmospheric tank is actually plastic but because of where it is tucked away it gets neglected (most don't even know it is there) and the spillage is what leads to the corrosion.
The tank serves no real purpose other than to keep things "tidy" and avoid coolant puddles under nervous owners cars but that is only ever an issue at that one time warm up.