My chain broke on my '84 300SD...because the rails were old and cracked. When cold (Dec.) the rails cracked and caused broken gears, broken valve cover, etc---$2500 worth of parts and labor. I would change the rails...they are cheap, and you don't know how old your's are.

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1983 300SD 343K everyday car
1983 300SD 285K from junk yard-tooks parts from deer car- runs great. Brothers car.
1984 300SD parts car-Hit deer
1979 300D 175K non-turbo "Doctor"
1979 300d parts car