Haynes say 20 Nm - 15 lbf ft for aux belt tensioner.

I have not ever broken a bolt off on an engine, and plan not too Sounds like too much of a hassle to fix.

 

I replaced the tensioner pulley today. It's still vibrating a tiny bit but nothing like it used to. I just think I need a new s-belt. It's definitely an improvement though.

Having that Torx bit makes it so easy to get the belt on and off. Its a T60 for anybody that buys the part.

 

One of the first things I bought when I got the car was a set of torx keys as I noticed how often they were used. Unfortunately for me, they stop at T50 :P

Thanks for the info though, now I know which torx key is slightly smaller than a 10mm Allen key