There are two through bolts that have to be removed, you may miss the bottom one. I think it is a 14 mm socket or wrench that will unscrew the tensioner to loosen it (in your picture). My altenator was a tight fit into the slot in the block, was not easy to move. You may have to force it up to loosen the belt after you remove the two bolts that hold the altenator onto the block. Move it in a way that will loosen the belt. The nut that you see on the tensioner will unscrew and can be removed from the screw. That bolt and the screw that acts as the tensioner is a $35.oo item. Good luck.

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1982 240D Euro Stick 4