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. I have a replacement cap, for the XK120, the AC RC5, 4lb. round, 72mm top , smooth-no tabs on the top.two lock tabs on underside lip of the top, it is 23.9mm (61/64in) on this one, from top of cap, to bottom of seal. The Rubber seal is fixed with in the cap base. Visible spring. AC Delco, Made in Dunstable Inland, on the top “Turn Down Tight with arrow beneath, and Remove Slowly” with arrow beneath: also on the top: RC5 K7 AC 4.
Important Note: some replacement caps seem to fit,…but are not deep enough to contact the neck seat. A deep reach (apprx 1 in) cap is needed. Some were 1 1/4 and likely too much. 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,