Seems to me that the general rule on the timing chain/guides is l00K miles and/or 10 years. From what I have been told the chains will stretch somewhat but seldom break. Most of the problems come when the plastic guides get brittle after all those years and break, dropping into the chain and that causes major damage to the engine. So again, 100K miles and/or
10 years

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1986 560 SEL (159K miles)