I know that the radiator cap on the 401 and 403 is the same, I assume the 400 and 402 would be as well, am not sure about the later 2 Litre cars without checking the parts book but I thought it might be worth mentioning that they are almost identical to the caps used on the MK VI and R Type Bentley and Rolls Royce of the early 50’s.
The Bentley Cap is a perfect fit in the Bristol radiator, they are not cheap at around £75 and you would need to order the rubber seal as well at around £3. They are available from Flyingspares, Introcar and no doubt a few other suppliers. If you have a good cap but bad seal I found those fitted my existing cap on one rad perfectly.
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