I have some old Mercedes brochures showing the engine assembly process...one of the pics shows them powder coating the insides of the engine block!

If MB has been doing it for years, it's probably okay. And the valve cover doesn't see NEAR the heat stress that the block endures!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car