Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 202Get this manual
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Forward Gear, Pinion Gear and Drive Shaft Installation
1Install FORWARD gear assembly into forward gear race2Install drive shaft through upper drive shaft bushing, oil sleeve and lower drive shaft needle bearing3While lifting drive shaft slightly, install lower thrust washer (GROOVED SIDE FACES DOWN) and PINION gearIt may be necessary to rotate drive shaft slightly to engage pinion gear with splines of drive shaft and with forward gear teeth4Install upper thrust washer above upper drive shaft bushing