there's different ways of doing an x-clamp replacement. people have their own preferences.
i use m5 .80-10mm hex cap bolts (you'll need 8 of them), 5mm steel washers and 5mm nylon washers. i use 8 of each.
i put the bolts through the motherboard, put 1 nylon washer then 1 steel washer on each bolt, top side of motherboard. nylon goes first to protect the board. add your thermal paste (make sure you have cleaned the chips first) and then attach the heatsinks.
if you have an xbox that was manufactured 2005-early 2007, you'll notice the 2 center standoffs for the case screws stick up higher. so if you were to put the board back in the metal chassis, those 2 standoffs would be pushing up on the center of the board, causing flex. so i shave those down. you dont have to use the 2 center screws. itll be fine. now the board will sit flat. the hex caps on those bolts are the perfect height for support, for the center of the board. you'll see what i mean.
hope this helps. good luck. oh... a good reflow will do wonders too! remember...it's an x-clamp
replacement, not x-clamp fix
