Primarily affecting the lower gearcase, corrosion eats away at the aluminum housing over time, causing porosity through the case. While this typically happens to the lower unit, it’s also common for the upper housing. In severe cases, the aluminum will actually be eaten away so dramatically that the innards (usually the propeller shaft bearing carrier, located inside the aft end of the “bullet” just in front of the propeller) are exposed. In other cases, lower unit lubricant actually starts oozing through aluminum.
A very common occurrence, impacts are typically from shallow water obstructions such as rocks, stumps, or oyster bars, or even dragging the drive on the boat ramp or road. Impact damage can usually be remedied; f the impact is just to the skeg (below the bullet), the damage can sometimes be repaired by welding or replacing the skeg. This must be done by a skilled welder who knows metallurgy well, because a poor or incorrectly performed weld can easily break. This is especially critical in high speed (55+ mph) applications, because at higher speeds, with a drive that is raised up higher for lower drag, the skeg takes on a lot of side forces not normally applied in lower speed craft. If the lower unit bullet is damaged, the damage is typically not repairable, and the drive should be replaced. In some cases, the impact damage occurs on dry land; the driver backs the rig into a tree, fence, another vehicle, etc.
Note: Whenever impact damage occurs, especially if it’s severe, the surrounding components (upper drive, transom housing, and the transom and hull) should be carefully examined for damage. It is easy and unfortunately common to replace the damaged drive but ignore the damage to the boat and other driveline components.
Older drives, especially older units used in saltwater and not flushed and serviced regularly, can soon become difficult if not impossible to service. They may be leaking oil, inoperative, even corroded beyond repair, and the service technician cannot get the drive apart without breaking parts—parts that may no longer be available. Some older drive parts are hard to find; for example, the older Swedish-made Volvo drives and out-of production drives such as OMC and Yamaha.