Datamax ST-3210 Operator's Manual page 38

Adjusting the Print Side Media Sensor –
Turn 'Off' the Power Switch. Open the Access Cover.
Loosen the Thumbscrew and adjust the Media Guides to fit the width
of the ticket stock:
The guides should be positioned so that there is no side-to-side ticket
movement (too loose), but not so close as to cause friction or bowing
of the ticket (too tight). (See Step 5 in Section 3.2.1 for an
illustration).
Once properly positioned, tighten the Thumbscrew to secure the
Media Guides in place.
Using a small wrench or nut driver (M3), slightly loosen the Nut that
secures the Media Sensor.
Nut
Print Side Media Sensor
Media Guides
30 S-Class