My WAG is your are using a standard glycol anti-freeze coolant in your system. Since your radiator situation is a non-pressurized system you could try
No-Rosin, which provides good anti-corrosion for the engine and since it is a water based approach you achieve better heat transfer.
On my '32 901 I had a problem getting the stuck valve and guide out. I made a "
Tool" to push both up and out.
Good Luck with your valve and head issue.
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