heater core

Alright, i finally got to the heater core and removed all the panels. I have removed one screw on the retaining straps using a 7mm socket, however i can not get any of the other ones off with 6-9mm sockets, or any of the Sae sockets. They all grab a little bit, and skip over. 11/32 _almost_ fits, grabbing better than any of the others, but still skipping off. What kind of screw is this? It looks like an octagon, with six sides.

Oh, by the way, Its a 1992 Oldsmobile Cutlass Ciera with a V6 an Air conditioning.

never mind. its 7mm. my socket just wont clear the tanks on the side. need a wrench :/