Yamaha Outboard Motors Factory Service Manual F6 and F8, Page 356Get this manual
BACKLASH BACKLASH NOTE:
Both forward gear backlash and reverse gear backlash be measuredIf both forward gear backlash and reverse gear backlash are smaller than specified, the pinion may be too lowIf either of these conditions exist, then check the pinion shim selection
MEASURING THE FORWARD GEAR BACKLASH
Measure: Forward gear backlash Out of specification Adjust