If you look under the block and to the right of the socket you will see the white nylon washer. Ontop of that is a standard nut. I do that keep the bolts in place when I take the block off so the bolts do not move around. Makes sliding a block on and off much easier. The way I have it there makes it to where you do not have to take the mobo out or any other mods to get the block off, and it makes it alot easier to mount the block as you can use both hands on the block. With the bolts anchored like they are side to side movement is minimized.