On the 6 cyl, you don't have to worry about it. The V-8's are the only ones that need the timing chains checked or replaced.
The reason for this is how long or short the chains are. On the v-8, the chain is over double as long as on the 6 and therefore, a little stretch equals allot when you get to the second cam. Also, the rails in the 6 are mostly made from metal rather than the plastic of the v-8.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS