That's a clamp bolt, I believer. Loosen it, and the tensioner rod nut should release.

Some of them have a star wheel on a bolt head -- release the nut on the BACK, then use the star head to tighten/loosen.

Make sure you're turning it the correct direction, too -- I stripped the one on the 280 attempting to remove the alternator belt going the wrong way -- it may not work exactly the way it looks.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!