That's interesting you mention problems with the late M117 (and presumably late M116) valve covers. Are they aluminium like the early engines or magnesium like the later ones? From memory, they are painted or coated suggesting magnesium. Do they suffer the same problem of the coating blistering and peeling like with the M102 & M103? The early M116 in my 350SLC has valve covers in aluminium without any coating. After 30 years they still look good and seal just fine. You mention having 7 thou taken off to make yours flat again. The cover from my 190E looks like it would need 70 thou off to make it flat again! The distortion cannot be caused by over-torquing the nuts in this case since the cover has spacers around the studs which bottom out, giving the correct crush on the gasket. Machining is not an easy option either since the cover has a groove to locate the gasket. The groove would need to be machined as well as the face. The spacers would also need to be machined a similar amount to allow the correct gasket crush.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.