Removing and XBOX 360 Heat Sink?

Ok so I am modding my 360 and having some trouble getting the screws out that hold the heat sink on, there are 8 of them on the back of the housing the MOBO is mounted to, I went out and bought an allan wrench set but for some reason the 5.6mm is slightly too big for the screw heads (I read during my research that, that was the aprox size). So can anyone tell me what the correct tool is to remove those? Its putting a nasty halt in my project...all I need to know is what tool to get, we have both a lowes and home depot by me so it shouldnt be a problem getting them....thanks in advance for the help.