T-50, I'm pretty sure, but maybe someone put some other size bolt in there. Also, I haven't done it myself, but some say there's a hex head that can be used to detension also, but I think that's on hydraulic tensioner on access. belt.
On my A/C tensioner there are two places to use a torx socket. There's a big one (probably T-60) on the tensioner arm and if you pop off the plastic pulley cover you can use a T50 or T55 socket in the pulley bolt. My torx socket set only goes up to T55, so I used either a T55 or T50 socket on the pulley bolt. Later I bought a separate T-60 Torx socket.
I've just jammed an allen wrench in there... thankfully there isn't a huge amount of tension so it turns without damaging anything. Someday I'll buy the right tool. But just as a big channel-lock can pop a fan off, so too can other tools be made to work.
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