1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 257Get this manual
NON BIGFOOT GEAR HOUSING
4Replace clutch if jaws are rounded or chippedRounded jaws may be caused by the following: Improper shift cable adjustment or linkageEngine idle speed too high while shiftingShifting too slowly from NEUTRAL into FORWARD or REVERSE
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: Splines are twisted or wornOil seal surface is groovedShaft has noticeable "wobble" or is bent more than 0.006 in(0.15mm)