I'm with hs_300e, a bad injector (or wiring harness) would cause the engine to run lean on that cylinder and would melt a piston.

You would see this on the spark plug you pulled out of this cylinder. If it was running lean, the spark plug would appear more burnt than the others.

Sholin

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What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.