12mm hex for the bolt through the tensioner. I've seen some come loose.

Sixto
98 E320s sedan and wagon

Also, replace the pulley no matter how it looks. When they're off angle like this, the pulley surface wears conically. If you don't replace it, you'll be back again. The pulley is generic, so there are lots of options for not much money. I've had good service out of Gates pulleys.

I'm going to post a link to an old thread of mine with pictures that shows what can happen when the pulleys fail at the sort of very gradual damage that can happen to the tensioner arm. I have since replaced my used part with a brand new part.

http://www.peachparts.com/shopforum/diesel-discussion/359649-om603-engine-swap-belt-use-plus-other-interesting-things.html