The vertical card compass is very sensitive to stray magnetic fields within the aircraft. If the compensators of the compass will not bring the compass
within tolerances, it will be necessary to install the PBB475 compensator ("balanced balls"). These balls contain magnets which permit the compass to be properly compensated.
Note: These compass compensators are made specifically for the Precision aviation compasses. The mounting holes may not match other compasses.
Per the manufacturer, this should work as long as the magnetic variation is not greater than 20 degrees. It is also recommended to use magnetic shielding with the compass compensator.
The compass compensator comes with directions for mounting this behind the compass.