Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 190Get this manual
4Replace clutch if jaws are rounded or chippedRounded jaws may be caused by the following: aImproper shift cable adjustment or linkagebEngine idle speed too high while shiftingcShifting too slowly from NEUTRAL into FORWARD or REVERSE Water Pump, Drive Shaft and Shift Shaft
1Remove bolts from pump cover
2Remove cover, nylon washers (above and below impeller), impeller and drive key from drive shaft3It is recommended that impeller be replaced whenever gear case is being serviced
5Inspect bushing surfaces of propeller shaft for pitting or wearReplace shaft and corresponding bushing if wear or pitting is observed6Replace shaft if any of the following exist: aSplines are twisted or wornbOil seal surface is groovedcShaft has noticeable "wobble" or is bent more than 0.006 in(0.15mm)