Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 197Get this manual
GEAR HOUSING
Propeller Shaft
1Replace propeller shaft if any of the following conditions exist: aSplines are twisted or wornbOil seal surfaces are deeply grooved; allowing water to enter gear housingcSliding clutch slot worndPropeller nut th reads stripped, damaged or excessive ly worn
2Check propeller shaft for straightnessPlace shaft on either balance wheels or "V" blocks and rotate shaft while observing spline area for bend (wobble)Replace shaft (if bent)