Take a look at this tutorial to learn how to install the X-Clamp mod fix for your Xbox 360. In this video, the instructor only ordered one set of washers and so when he accidently drilled the wrong side on the metal case, he was forced to improvise. Instead of using 4 screws and those washers for one heatsink, they were divided between the two because the heatsink for the CPU wasn't tight enough at all without them. So 2 screws were installed in a diagonal direction for the GPU and 2 in a diagonal direction for the CPU. So pretty much each heatsink was only attached by two screws.