Big high resolution herringbone gear with true 12.8 mm hexagon bolt head socket. I've also added a second version with a 13 mm socket, just in case someone needs that little bit of clearance. Some people have been pointing out that the previous version http://www.thingiverse.com/thing:97377/#files has a hexagon bolt head socket hole that is 14 mm wide. Somehow that ended up fitting my M8 bolts just fine, even though my bolt heads are the standard 12.8 mm wide.