May be an obvious answer, but can you get a set of Vicegrips on the head? I second pounding a Torx into it. 
       Or cut a slot in it and try a big screwdriver .

                                Stacy 

Sent from my iPhone

> On Sep 20, 2015, at 4:36 PM, William Monk <billmonk@GMAIL.COM> wrote:
> 
> You could find a socket that is just a bit bigger than the Allen bolt.
> Pound it over it to remove. Another way is use a torx bit that can be
> tapped into the Allen hole.
> 
> Bill