Those rails were white when they were put in and have become discolored. They are cheap, so replace them. In addition to them, while you have the engine out, I'd replace the timing chain and tensioner. The camshaft sprockets don't appear to be that bad. Mine were worn to the point the tips were bent over at an angle--very close to a catastrophic failure.

There are plenty of directions on how to roll a chain in available here. If you don't have the right tool to take the retaining pins for the rails out, you can manufacture something by finding a bolt that will screw in to the pin or find a small puller to do that job. Only thing to worry about with the sprockets is which side goes toward the chain. Can't remember right off the top of my head.

Paying retail prices, I'd guess the parts would run about $500.00. Not cheap, but not too expensive either. Of course my frame of reference is the complete engine rebuild that I just completed awhile back.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000