The lower unit (LU) on outboards house the water pump and gearbox as well as supporting the thrust bearing for the propeller. There are several seals that can go bad and necessitate LU removal. Water pump maintenance also requires removal and will typically be done at least once a season. Most motors will mount a sacrificial zinc trim-tab onto the LU under the anti-ventilation plate to correct for wheel-walk and to protect against corrosion.
Dropping the Lower Unit
Drain all oil from the gear case. Remove the fill plug and the drain plug and allow to drain completely. Inspect the oil as it comes out. If it looks like milk or peanut butter in color then you have a bad seal that must be replaced. Disconnect the shift linkage at the top end near the powerhead.
Remove the mounting bolts located under the LU flange. Do not force the bolts if they are held fast by corrosion. Soak with penetrating oil and try again. Remove the trim tab plug from the top of the anti-ventilation plate and unbolt the trim tab.
Place blocks under the LU to prevent an uncontrolled slide to the floor. Slide the LU down off its locating pins. Lightly tap on the LU and the motor leg with the soft hammer to overcome any drag from corrosion. Heat may be used to separate especially stubborn joints. Use a torch to heat the LU near the joint. Allow the metal to cool and repeat several times. The expansion and contraction of the LU metal may overcome the friction and break loose. Slide the LU down off of the driveshaft.
- Be careful to limit the use of heat to near the LU/motor leg joint. The LU oil could reach the flash point from the heat of the torch and ignite.