Which radiator cap goes where?

I have a cap marked "16" and another "20". Does the "20" go on the radiator and the "16" on the reservoir?

Thanks....Karl

 

The 16psi goes on the wing header tank. This is what controls the entire system pressure. The 20 on there will over-pressure the system, but it probably does not matter.
The crosspipe spout cap, if you look at its overflow, vents into the radiator-top air purging pipe - which is itself still at the system pressure controlled by the wing header tank. So it does not affect system pressure if it lifts or not. In fact, a blanking cap is best on the spout, with no sprung inner! If no blanking cap, then I would use 16 psi in both. Which is what Jaguar did on later models, or eliminated the spout on the crosspipe altogether - but you need a header tank higher than the engine to do that.

 

 

Item #1 in this illustration, what you're calling the reservoir.:

https://www.jaguarclassicparts.com/u...ipes-5-3-litre

Cheers
DD

 

front left hand corner of engine bay. Header tank also called expansion tank.