1998-2006 Yamaha F20/F25 Outboards Service Manual, Page 251Get this manual
! SHIMMING (FOR USA AND CANADA)
4Select: Forward gear shim(s) Remove or add shim(s)Calculated numeral More than 1.00 1.10 1.20 1.30 1.40 or less than 1.10 1.20 1.30 1.40 1.50 Shim size to use 1.0 1.1 1.2 1.3 1.4
Available shim thickness: 1.0,1.1,1.2,1.3, and 1.4 mm
Reverse gear shim