The plastic parts do get old and brittle and are prone to breakage. Seems that 100K miles and/or 10 years is the replacement guideline. As long as you are doing the guides you probably should replace the chain too. There are not too many reports of the chains actually breaking on their own. Usually the problem is the plastic parts break and lodge in the chain causing the chain to jump time and cause major engine damage. But since the chain will be easy to replace at the time you are doing the guildes you might as well do the chain too....

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1986 560 SEL (159K miles)