Radiator cap 120 early 140

radiator%20cap%20oriinal%20AC%20RB5%204lb%20no%20seal%20underside radiator%20cap%20oriinal%20AC%20RB5%204lb%20no%20seal 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,

Same as my mk9, and also used on Rovers of the same vintage. It seals on a large fiber washer which sits in the filler neck which comes with it.

Here’s an interesting cap. It was on my XK120 when I bought it in 1980.

There is no maker’s name, but it is probably Stant, since the first patent number stamped on it was owned by Kyle Stant and assigned to Stant Machine Co. The patent dates from 1928 and is for the locking tabs and the rotating thin disc sealing design. Before that all caps had machine threads and sealed with a gasket.
The second patent number stamped on it owned by John Karmazin of Detroit dates from 1925 and is for the pressure relief design. Before that radiators were not pressurized and needed frequent refilling.

Here’s the old cap that was on my late 140 when I got it. The cap is an AC.

It still works, I now think I will try to gently remove some rust and clean it up a bit. Looks like it may have somehow gotten some black paint on it.


The original radiator pressure cap of the XK 120 is in fact a further development of the version used on the WW2 Jeep and other US military cars. The US army did everything to standardize this type of cap for as many as possible cars during the war and “invited” all US radiator cap manufacturers to supply this type of cap. See this photo:

After the war this pressurized cap type was available in large quantities in the UK and the British branch of AC took over production and hence Jaguar used them on the XK 120 and Mk VII.

There is so much more to tell about this type of radiator cap (including all the mistakes that have been made in the past when restoring these XK 120 radiators), which I will do as soon as I’ve moved house, as all my time is presently involved in this.

Bob K.