The other reason why the 617 engine runs for so long may be because it's all cast iron. Of course I am referring to the block and head. Whereas later they went with aluminum heads. Why MB got away from such a proven "bullet-proof" design, I don't know. Diesels run cooler and slower. BMW's gas engines are racier compared to MB's gas engines, but which one last longer? How many early 80's BMWs do you see, compared to MB.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member