1996-2005 Suzuki DF40, DF50 Four Stroke Outboard Service Manual, Page 276Get this manual
9-23 LOWER UNIT
LOWER UNIT GEARS- SHIMMING AND ADJUSTMENT
If lower unit has been rebuilt or has had components replaced, shimming for correct gear contact and backlash will have to be adjusted to ensure smooth, reliable operation of gearsShimWasherMounting position