That's PRECISELY the kind I have in my toolkit. Now, if THAT one don't work. There's something wrong. Can you shine a light down in that hoie and see the hex head, and the top of the cylinder head? I'm thinking that perhaps, though unsure as to how, that hole got all filled up with crap and you are unable to get the socket on the hexhead FAR enough to apply the torque necessary to break it loose. Could be, sockets you are using are not long enough to do this. Beacuse it IS a pretty deep hole, and tight quarters as well. Perhaps a "thin-walled" socket (like the last one Shep linked to) is in order.