Unless it changed for `06, the allen bolt threads into the bracket. The hole in the bracket is threaded, there would be no nut on the backside. If it was a rivet, it wouldn't have an allen head on it anyway. Sounds like you'll have to drill it out.

The others are referencing an allen socket, available at most tool outlets. Looks like these.

http://s7.sears.com/is/image/Sears/00934448000

 

OK - gotcha. It's threaded.

And I just bought the hex ratchet set from Sears that you referenced. The "correct" size now slips, and the next larger size is too big.

Need to drill it out.

 

man, you didnt know that you could get a torx or allen socket to go on your ratchet??????

 

Try the sears bolt-outs as recommended before. Heat the head a little with a propane torch (lightly) before you apply torque.