Brother MFC9100C Service Manual page 123

(6) Put the gap reference gauge on the surface plate.
Check the small dials to see that each needle points to the vicinity of 1. If either of the
needles deviates significantly from 1, turn the corresponding vernier dial. One turn of the
vernier dial rotates the needle of the small dial by one graduation.
Press the gap reference gauge against the reference plain of the surface plate and then turn
the two vernier dials to make each needle point to zero (0).
IV - 82