I was in Napa a while ago buying some Zerex and asked about bolts.

The guy pointed out that there are grades printed on the package or notches cut on the head of the bolts to indicate whether they are case hardened or not (or something to that effect).

Grade 5 (3 notches) is about average where as Grade 8 is case hardened (or some such thing) for high-stress applications.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black