I got my new MB radiator cap from the dealer a week or so ago. It is built much better than the auto part store cap. $12 bucks or so. The "old" one is in the trunk as a spare.

Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.