5", the CAD people didnt pay attention while designing. Our work around is we added a corner bracket on and we are mounting the brushes to that (im pretty sure it screws straight into the aluminum tslot)
Unfortunately I dont have a picture of it, but I know for a fact it is held very securely.
Basically one of these for t-slot (they have all tslot parts in a pdf and odd page view) (with the gussets)
http://www.8020.net/HT-Series-15.asp
And actually we are right at the weight limit with that much T-slot and the easy brush adjusting (except for chains) comes in handy.