Asus PSCH-SR/SCSI - Manual (Page 28)

2-10 Chapter 2: Hardware information
2. Hold down the heatsink lightly
and twist each of the four
screws with a Philips (cross)
screwdriver just enough to
attach the heatsink to the
motherboard.
3. When the four screws are
attached, tighten them one by
one to completely secure the
heatsink.
Do not overtighten the screws to avoid damaging the motherboard.