from Roger Payne post: Radiator Cap: (thank you RP)
The correct XK120 and early XK140 cap is a MADE IN ENGLAND AC Model RB5 , a Circular Top, a closed (shrouded pressure release spring) with no sealing washer, with a one inch depth valve seat, in conjunction with a seal/gasket mounted in the flat of the radiator neck, and also rated at 4psi.
The later type is a MADE IN USA AC Model RC5 (USA AC Part No 850799).also a circular top, but with an open/visible larger spring loaded seat with integral rubber washer, with a 7/8in depth to valve seat, and rated at 4psi.
Important Note: some replacement caps seem to fit,…but are not deep enough to contact the neck seat. A deep reach cap is needed. Do not exceed the 4lb. The thermostat can be damaged. If you want to have a higher boil temp just make the anti-freeze a 60-40, or 70-30…but keep the 4lb cap. But…it is quite high (223F at sea level) already at 50-50.
photos of an original Made in England RB5 original which has no internal rubber seal…the separate seal inserted into the radiator neck must be used.
and per RP: The actual original cap used on a 3.4 litre XK150 is a C.10289 which is also rated at 4 psi, and originals were made by AC (UK)
and BTW…the normal coolant level for the XK120 is an inch or so below the neck. Jaguar issued a Service Bulletin about this. If you fill beyond that , some coolant will be expelled: what you end up with is the normal level. So don’t keep topping up…and wondering what is wrong,