Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 198Get this manual
GEAR HOUSING
Drive Shaft
1Replace drive shaft if any of the following conditions exist: aSplines (a) are twisted or wornbLower drive shaft need le bearing race su rfaces (b) are pitted or wornReplace corresponding bearing as wellcOil seal surfaces (c) are deeply grooved or pitted allowing grease to wash off crankshaft splines or water to enter gear housing