Yamaha 115-225 HP Outboards Service Manual, Page 164Get this manual
Reverse gear shim 1) Find reverse gear shim thickness (T1) by selecting shims until the specified measurement (M) with the special tool is obtainedSpecified Measurement (M)25_30 mmFI100 mm (LV4) 29.10 mmF100 mm (LV6)
If the value is negative-), then subtract the value to the measurement2) is the deviation of the lower case dimension from standardIt is stamped on the trim tab mounting surface of the lower case in 0.01 mm unitsIf the mark is missing or unreadable, assume an mark of "0", and check the backlash when the unit is assembled