MSI ATX BX1 User Manual page 10

Step 3: Install the Heat Sink Support Base.
Look for the Two holes across Slot 1, and match it with the Two legs
of the Heat Sink Support Base for the proper direction. Take note
that one hole/leg is bigger than the other. The Four top pins of the
Heat Sink Support Base should also be oriented towards Slot 1.
Leg
Push the Heat Sink Support Base onto the mainboard, until you hear
a click sound. Check for a perfect fit.
Step 4: Install the Heat Sink Support Pin.
Push the Heat Sink Support Pins onto the two holes of the Heat Sink
Support Base. Check for a perfect fit. These pins are used to secure
the Heat Sink Support Base.
pins
Heat Sink
Support Base
2-5
Heat Sink
Support Pin