In the hole where there is supposed to be an 18mm bolt, there is no bolt. So it either worked its way out (vibration?, improper torque?, no red lock-tite?, other?) or it is sheared off......... If it is sheared then the engine is going to have to come out and then crankshaft is going to have to be replaced. That's beyond me and my checkbook.
What if the bolt just came loose and backed out? Does that happen? Wouldn't I have had some signs that the bolt was coming loose, like pulley or belt noise or vibration?

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"Frau Blue Car" 1980 240D think Frau Bluker, cue the horses, formerly known as "China" from JimmyL
"Gunther" 1982 300D.....Turbo!! A/C!! Sunroof!!
"Fraulein" 1975 240D----Donated to NPR