Originally posted by neileg
Im 99.99% sure they are not magnesium. Too expensive and difficult a metal to use on such a routine part.
I believe they are aluminium that is powder coated. Where corrosion gets under the powder coating, the aluminium turns to a powdery oxide that could suggest that its magnesium, but it's not. Your SLC covers will be anodised.
I've sold my 190E so can't look at the cover. The spacer system you describe should surely stop warping of 70thou. I'm not arguing with you, it just doesn't make sense the way you put it!
The MB North America M102 shop manual definitely states that the cover is magnesium. Some earlier 190Es got a plastic cover that was even worse. They recommend replacing the plastic cover with the metal one. The extremely light weight of the cover also tends to confirm a magnesium alloy. At the price of a replacement I decided against trying to see how well it burns to confirm whether it is magnesium! The cover on the M103 in our 300TE appears similar, although it is still in reasonably good condition with only a couple of blisters in the coating. I maybe incorrectly assumed the black finished covers on the engines of mid '80 onwards all used magnesium. These covers have areas with a rough finish under the black coating, unlike the all-over smooth finish of those on the MB engines of the '60s and '70s.
Checking my M102 valve cover with a straight edge confirms up to about 2mm (0.079") variation along the gasket face on the side with the scallops for the spark plugs. The sections without the mounting holes sit that much higher than those with. The opposite (inlet) side which should be straight also has an obvious bow in the middle. On checking, it was found second hand covers were in no better shape than mine.
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.