I recently picked up a 1990 Mercury 20 hp. motor and it's in really good shape but the lower unit is junk (was run with no oil in it). I have a 1983 Mercury 18 hp lower unit that is in mint shape. The 18 hp lower unit had different size bolt holes to hold the lower unit onto the motor, was able to enlarge them. The lower unit mounts up no problem now.
One thing I noticed between the two lower units is on the 20 hp there is a notch at the top of the drive shaft (see attached photo). There was no O ring or anything in the notch when I took the lower unit off. Does anyone know what the notch is for? Would the drive shaft on the 18 hp be compatible or is the notch required?
I also noticed there are other differences as well, the gears have slightly different part numbers. eg. the reverse gear on the 18 hp is 43 99542A1 and the 20 hp is 43 99542T. Does anyone know if that will be a problem? Plus the drive shaft from the 20 hp is held in by a bolt and the 18 hp just pulls out (no bolt). So I don't think I can swap just the drive shaft.
I'm mostly worried about the notch at the top of the 20 hp lower unit, is there suppose to be an O ring on it? Is the notch going make a difference or not. Any information is greatly appreciated.
One thing I noticed between the two lower units is on the 20 hp there is a notch at the top of the drive shaft (see attached photo). There was no O ring or anything in the notch when I took the lower unit off. Does anyone know what the notch is for? Would the drive shaft on the 18 hp be compatible or is the notch required?
I also noticed there are other differences as well, the gears have slightly different part numbers. eg. the reverse gear on the 18 hp is 43 99542A1 and the 20 hp is 43 99542T. Does anyone know if that will be a problem? Plus the drive shaft from the 20 hp is held in by a bolt and the 18 hp just pulls out (no bolt). So I don't think I can swap just the drive shaft.
I'm mostly worried about the notch at the top of the 20 hp lower unit, is there suppose to be an O ring on it? Is the notch going make a difference or not. Any information is greatly appreciated.
