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Stern Drives         The inspection of a stern drive unit can be very difficult, or impossible if the vessel is in the water.  If the vessel is not hauled for a complete bottom inspection which would include the stern drive, a lot would be left to chance.  Often, during the sea trial the stern drive can perform very well and still be on its last leg. A stern drive can perform reasonably well right up to the point where it is about to fall apart.  Number one, the outside casing is made from aluminum and the inside parts, gears, shafts etc. are made from various other non corrosive metals. Electrolysis plays a very important role in the condition of a stern drive. A stern drive can look very good from the outside, but the inside may be an entirely different story. Usually the condition of the outside casing can give you a pretty good idea of the condition of the inside. Most stern drives that have been poorly maintained will show excessive corrosion. The casing literally turns to dust over time. Typically a drive will be equipped with an array of zinc anodes which should be monitored frequently during the season, as they tend to disintegrate at various rates.       There are also several grounding wires within or attached to the drive that must be routinely checked. If the ground wires fail, electrolytic action is enhanced.         I had the good fortune to inspect a vessel with twin stern drives a while back. One drive looked fairly new and the other was badly corroded. The owner said that he recently had the nice looking one rebuilt and a new casing installed. I noticed that he surface was not quite as smooth as it should have been if it was, in fact, a new casing, so I scraped a little paint off in a couple of the rough areas. Not to my surprise, I discovered areas of epoxy filler. Upon closer inspection I found that the entire lower unit had been re-finished, filled with epoxy and sanded reasonably smooth, and painted. The lower unit had not been rebuilt mechanically, only cosmetically. It turns out that the owner did the job himself to hide the massive corrosion problem and was about to do the second drive when his time came for spring launch. This is another example of the seller of a vessel misleading the buyer.