I second this scenario, its happened on both my 85 300D and my 83 240D. If the alternator isn't installed correctly, with the correct belt tension it will continue to work, possibly for thousands of miles. The oval-ing of the holes I think is a result of mis-installation, and/or from rebuilt altenator with slight variations in the locations of the holes. Eventually there is enough slop in the alternator for it to wedge itself at an angle other than 0 deg to the pulley and this is when the bottom bolt can snap.

Also I've snapped replacement bolts that were non OEM cheapo advance auto bolts on that lower hole. Probably because it has machined threads and a combination of overtorqueing and insufficient hardness on the bolt.

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1985 300D Gretchen (Astral Silver) 220k
1983 240D 4-speed Evelyn (Orient Red) 203k


TANSTAAFL