Most MB engine experts will tell you that 10 years and/or 100K miles is a good timetable for replacing the timing chains and related parts. The chains seldom break though they do stretch a bit. However the plastic guide rail pieces become brittle with age...and if they break then you are looking at major damage to the engine if the plastic piece falls into the chain and the chain jumps time.

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1986 560 SEL (159K miles)