NEC Express5800 Series EXP320B - Manual (Page 339)

System Upgrade 8-27
8. Place the CPU on the socket carefully.
TIPS:
Pay attention to the orientation of the CPU. The CPU and
socket have pin marks to prevent misinsertion. Check the
pin marks of the CPU and the socket and insert the CPU
correctly.
Pin marks
9. After pressing the CPU softly against the socket, close the
lever to secure it.
10. Remove the film of the cool sheet on the heat sink.
11. Place the heat sink on top of the CPU.
12. Secure the heat skink with the screws. Firstly, fasten four
screws in a crisscross pattern tentatively, then fasten the
screws tightly.