1998-2006 Yamaha F20/F25 Outboards Service Manual, Page 247Get this manual
SHIMMING (FOR USA AND CANADA)
5Adjust: Shimming gauge-to-pinion gear clearance Remove or add shim(s)Available shim thickness: 0:7, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, and 1.6mm
Forward gear shim