I think the accepted time frame for timing chain and guide replacement is
100K miles or 10 years. Seems to me that the chain itself seldom fails but the associated parts which are plastic get very brittle after time and when they break they drop into the chain, the chain jumps time and that is when the damage to the valve train occurs. At an indie MB shop you are probably looking at about $800 to $1000 for the update.

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1986 560 SEL (159K miles)