tensioner assembly BROKEN!!!

[tylerf350]

I have a 2000 F350 7.3l Diesel with a bad tensioner assembly. I went and got a new part, but I can't get the old one off. I know it is one bolt in the middle of the tensioner assembly, but I don't know what size and I can't get anything to fit (it is all done by feel because I can't see the thing). If anyone has any advice or help, I would really appreciate it. Thanks

 

[Frankenstien]

Moderately certain its a 14mm.... but if not13/14/15 are the most common for under the hood accessory sizes, then its a jump down to either 10mm or 8... or a jump up to 18-24