Took my 1980's merc 15 hp to a local marina (which will remain nameless for now...I bet Brian could guess)2 years ago for a tune-up. Motor ran fine that season but at the end of the year as I drained the oil from the lower end I noticed water had gotten in, and that the upper screw came out real easily (ie housing stripped). After changing the oil I put the screw back in place and siliconed over it. This past season the motor ran fine again, but in the fall while changing the oil I noticed water had gotten in again. Marinas tell me to change the lower unit housing at a cost of around $500 - not gonna happen on a motor that's over 20 years old. Any suggestions? Anyone want to buy it and tinker with it themselves shoot me a pm!.