This is the eroded original radiator cap, replaced with new, kept for posterity.
I’m unfamiliar with its design.
There seems to be an inner coil but it isn’t sprung like the modern replacement.
How does this thing work?
Yes, the coil spring and relief valve disc are inside those six holes. The pressure pushes up the disc inside against the spring force. There must be something to allow the air (or coolant if you have a catch tank) back in when it cools off, perhaps just poor sealing on the disc.
Mine came to me with this one.
There are patent numbers on it.
1761396 (Karmazin 1930) & 1806224 (Stant 1931)
Karmzin’s relates to the pressure release.
Stant’s is about the bent clips that hold it in place, where most caps of that era had a machine thread.
So the coil expands with heat? It seems mine is missing the rubber seal … that answers another part of the question.
No, nothing to do with heat, just pressure, the coil you see is an ordinary spring, and the pressure on the disc inside pushes it up.
Do you understand the concept of pressure?
Like in your air compressor.
The hot coolant builds up a pressure in the header tank.
Say if your cap is rated for 4 pounds per square inch as mine is.
The area of the disc (pi R squared) is maybe a square inch.
A force of about 4 pounds on the disc should begin to compress the spring, to open it.
Yes.
There’s no coiled spring action in this cap. There’s a disc spring on the underside otherwise the assembly is sloppy.
Sounds like it’s brokey-wokey.
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And I guess that answers the question why I can’t understand the mechanism.
Thanks.
Nick,
Wrote a complete article on radiator Caps for XKs a few years ago. The XK 120 had the RB-5 cap type, in fact a direct derivative of the type introduced in WW2 on Jeeps etc. May be this “exploded view” will explain how it operates: one valve for over-pressure and one valve for vacuum relief after cooling down.
You absolutely need the fiber washer at the bottom of the filler neck of the radiator, otherwise the cap doesn’t work. If you need more info. just let me know.
Bob K.
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Thanks, Bob. It’s apparent the valve spring in mine must be broken.