Threaded Radiator Cap

I am looking to find a replacement radiator cap for my 1968 2000 TC manual car. The radiator in question has a non relieving cap on the top of the radiator (chromed) and a relieving cap on a tank on the side of the radiator. When they made those caps, they only included a few threads on the cap, even though the radiator has at least 3-4 times as many threads. The result is that 50 years on, the threads on the cap are worn to the point that they will not engage enough of the threads in the radiator to keep things from leaking. Has anyone else out there experienced this issue and found a solution? I have looked high and low to find a serviceable used cap with no success. I have talked to a radiator shop about modifying the radiator but all of their solutions would be higher than the existing cap and there is not enough clearance between the cap and the underside of the bonnet. I have yet to figure out how to successfully attach a photo but my radiator is the same as the car in the article at this link Thoroughly Modern Auntie – 1968 Rover P6 2000TC


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If it's only to fill the radiator, why not just have a boss fitted and fit a modern screw plug i.e. like a drain plug but at the top? That would surely not add any height. I had a boss fitted for a at thermoswitch and this would easily be wide enough to fill the radiator with a funnel or hose.

I'd remove both and fit a pressurized expansion tank and add the boss as a filler.


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I wouldn't modify the rad in that scenario, I would modify the existing cap. Is there enough length to cut more thread in the cap? If not it's not a big deal for a machinist to remove the old threaded portion, weld a new (longer if you wanted) section of tube on and thread it to match rad neck. That way it would still look original too.


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Hmm machining one from scratch in aluminium or brass would make more sense then I guess.
Didnt the very early 4 cylinder cars have brass caps? Have I imagined that?


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Hmm machining one from scratch in aluminium or brass would make more sense then I guess.
Didnt the very early 4 cylinder cars have brass caps? Have I imagined that?
they were made from a soft alloy, Mazak or similar and anodized so were a sort of brass colour
It is a cast white metal cap so any repair is difficult. The irony is that they included a hex nut into the cap so a person could really tighten the cap down and hence, easily strip the very short threads. Someone managed that long before I had the car. I am having a machine shop build a new one out of brass with a knurled diameter for hand tightening. A viton O ring should keep the seal. I will have them make the threads longer as there is plenty of room in the radiator filler neck. It will look different than the original, but function is more important than look at this point.
No need for a Viton O ring, a regular Buna-N will be fine, and much cheaper. You can use an O-ring with the original cap, the only problem being the original seal was a flat ring, so if the O ring is fat enough to fill the groove, you can't engage the threads and if it's smaller it can slip around and not seal unless you're careful. At one time I used I think three orings of increasing size and glued them to a cap and that worked well, but the last couple of time I cut a flat seal out of sheet rubber. None of that will be a problem if you're having a shop make a new cap of course.

I used flat sheet rubber for a while as well. After a shop I had doing some work stripped the threads even more, I could not make the rubber sheet work and switched to a very thin O ring. That has worked for a while but still takes a lot of down-force to get the threads to engage. As part of my major restoration project, I decided to address this. Thanks for the note on the O-ring material. The machine shop is going to throw in a viton O Ring this time around for free but I will go Buna N in the future. Thanks for the suggestion.