Please note that there are two options when installing a belt with this unit. You can use either a 12 point 17 mm box wrench/socket or a T-60 Torx head when installing the serpentine belt.
If you have had premature failures of this tensioner or the serpentine belt has broken or been thrown, you most likely have a failed alternator pulley. The alternator pulley is a decoupling pulley and has a one-way clutch mechanism to minimize belt flutter. Belt flutter places stress on the entire belt drive system and the tensioner acts as a spring-loaded shock absorber. If the decoupling pulley has seized, it will greatly shorten the life of the belt tensioner and often causes the pulley to break off or a fracture of the pulley arm. Replace the alternator pulley or the alternator to solve this problem.
Video demonstration of why the decoupling pulley is a critical component:
There is a lot of very good and easy to understand information here.
http://www.gatesprograms.com/adp Here is a second video: