Header tank cap XJS V12 86

Hi All,

When checking the cooling system I found that the two caps (one on the filler / crossover pipe, one on the header tank) are different in size:

The rubber (that should close the pipe) on the header tank measured ~35mm in diamer:

The one on the filler / crossover pipe measures ~40mm. There’s a lot of info and discussion on the pressure ratings. General opinion/ conclusion is that the one on the header tank should be 15 or 16 PSI and the pressure rating for the cap on the filler pipe is not relevant, as long as it has a rubber ring in the bajonet: it is only meant to close of the filler neck completely. Also the cap on the header tank needs a rubber ring in the bajonet: the idea is that cooling fluid can be recovered from the overflow tank.

The second picture shows what i think is a problem in my cooling system. The 35 mm diameter rubber ring doesn’t seem to close the header tank. Blowing air in the cooling system or just running the engine results in air comming out of the overflow tube (i’ve installed a transparent piece of tube).

So… I’ve ordered a replacement part from my local car-parts shop, according to their data gates part no. RC116 is a right replacement… I received the cap but it has the same 35 mm diameter and no rubber ring in the bajonet. So can’t be right…(?)

Questions. What’s the diameter of the closing rubber ring supposed to be? Should 35 mm be enough to seal? Or is the 40 mm ring the right one? Anyone has a part number that works? Apparetnly a cross reference to the original part number CCC6707 is not a quarantee

Swap em and see what happens.
Big quality differences in radiator caps…

Also make sure the locking ears aren’t bent to result in looser fit when l the way turned and locked

Evert, I have several brand new filler necks from a previous Jag project, so I dug them out and measured the hole where you pour in the coolant. 30mm, all of them. A 35mm seal might seal- just.
I would prefer a 40mm seal, personally.
The radiator cap needs two rubber seals to ensure coolant will be recovered from the overflow(atmospheric) plastic tank, and as Jim says the ears and top of the header tank cannot be distorted, or you’ll have a poor seal. The header should be filled to the top, BTW. As should the central filler neck.

Dave, that is interesting about filling to the top at header tank. I see the ROM says it that way at page 26-1.

I have always been told fill through center cap with bleed screw open and header tank cap off. When coolant visible at bleed screw shut it. Continue to fill slowly at centercap till header full. Cap it. Fill till center cap level 3 inches from cap seat. Thereafter always check at center cap so you don’t break the seal between header and atmos tank.

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Jim, my coolant crossover pipe started to leak several years ago, so I made a new one from copper tubing. (And a spare.) All that helped me get a much better understanding of how the complete cooling system is supposed to work, especially getting the air out.
If everything is working properly, the air ejection plumbing will push any air entrapped into the header tank, so that as the coolant heats up and expands, the air and some coolant escapes to the plastic overflow container that is hidden behind the LHS front wheel.
When the engine is shut off and cools, coolant returns to the header tank, but not the air.
If the header tank is not full, and contains some air, then the air ejection just has more work to do until the header tank is completely full.
The center fill tube should expel air also until it is full to the small tube near the top.

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I will try this to see if there’s any leaks. Swap the caps, blow some air in top of cooling system by removing a hose and plugging one end with manometer to see if pressure stays, But i wil need a right spec cap for the header tank, cooling system is supposed to operate at 15-16 psi, not 20…

Yep, this is what i learned as well about operation of the air removal system. Always wondered how the water could be sucked back through the header tank cap. It closes at 16 PSI positive pressure, so how can it be open at negative pressure??


Thanks Doug
Now we have trhe full story, didn’t see this picture before