Just to let owners of an XK 120 know, that when you’re looking for a new RB5 Radiator Cap, similar to the original version made by AC, you can use the version that AC USA made for the (Willies) Jeep from 1943 onwards.
These original versions have a very special construction with two separate valves for over- and under pressure. These caps require a fiber washer in the neck of the radiator opening. Modern replacements have a single spring with rubber seal (like all later radiator caps on the XKs) and require the fiber ring to be removed.
My original RB5 cap started leaking (see pic 1). I still had another one which is a somewhat later version made by AC UK (see pic 2) .
Still wanted a spare one and ordered the “Jeep” version, original manufactured by AC USA under part number AC 850091. These are mechanically identical to the British version, although the lay-out of the text is a bit different and the US version has the patent numbers mentioned as well. But all in all, an acceptable replacement for the original Jaguar XK 120 version. See pics below; you can “zoom-in” for smaller details.
One small note: some of these re-manufactured caps require the lips for the “bayonet” connection (see picture 3) to be bend “outward” to properly fit the Marston Radiator filler, but can be done.
Prices of these “army versions” are very reasonable: between € 15 and € 25 (and possibly even lower in the US).
Interesting, never seen those fibre washers before, I always wondered how they got a proper seal. I used an O ring on the Mk 7, it made snug fit in the neck opening.
The RB5 cap was later replaced by the RC5 cap, but never used on the XK 120. See also the instruction leaflet from AC that when using the new RC5 radiator cap, the fibre washer had to be removed.
Try searching “Willys Jeep Radiator Cap washer” or “Ford Jeep Radiator cap washer” or similar.
I can order them here in Europe for €2. But I know these must be coming from the USA , so you’ve to move away from Jaguar parts and aim at WW2 car part enterprises. The same cap was also used on other cars than the Jeep.
Next to AC, there was also a version made by Stant, another major supplier of radiator Caps in the USA. (Stant GPW-8100-A). Stant also supplied to Ford, even in Ford branded boxes. They must be out there somewhere…
