IBM X23 - Manual (Page 59)

Turn an additional 180 degrees after the screw head
touches the surface of the logic card:
180 degrees more
(Cross section)
v Torque driver
If you have a torque driver, refer to the ″Torque″ column
with each step.
v Make sure you use the correct screw. If you have a
torque screwdriver, tighten all screws firmly to the torque
shown in the table. Never use a screw that you
removed. Use a new one. Make sure the screws are
tightened firmly
Retaining serial numbers
This section includes the following descriptions:
v ″Restoring the serial number of the system unit″
v ″Retaining the UUID″
Restoring the serial number of the system
unit
When the computer was manufactured, the EEPROM on
the system board was loaded with the serial numbers of
the system and all major components. The numbers need
to remain the same throughout the life of the computer.
If you replace the system board, you must restore serial
number of the system unit to its original value.
Before replacing the system board, save the original serial
number by doing the following:
1. Install the ThinkPad Hardware Maintenance Diskette
Version 1.60, and restart the computer.
2. From the main menu, select 1. Set System
Identification.
3. Select 2. Read S/N data from EEPROM.
The serial number for each device is displayed. Write
down the serial number of the system unit, designated as
follows:
20:System unit serial number
Chapter 3. ThinkPad X20, X21, X22, X23, X24 55