Oki Microline 520 Service Handbook page 106

as follows. (Figure 2 B)
Turning the cam CLOCKWISE will increase the printhead gap.
Turning the cam COUNTERCLOCKWISE will decrease the printhead gap.
Verification Phase
13. Check the printhead gap adjustment at the left, right, and center of the platen.
14. If the printhead gap is not the same at all three positions, return to step 6.
ML520/521 ( 96-02-04 )