you're completely correct about cooling being provided by contact with the head. Everyone seems to say that the valves tighten over time but I'm sure they can also become loose.

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1969 230 inline 6 dual carb 60,000 Miles 2nd Owner doesn't run right
1982 300 SD 455K - my reliable commuter car
1990 190E needs new crank shaft ran it out of oil
1996 C220 bad wiring harness