The details vary from motor to motor, but here's a general description of the procedure:
Disconnect shift rod, remove lower unit retaining bolts (including possibly the torque tab and one or two bolts up under the antiventilation plate, depending on your model). Pull the lower unit off. Four bolts hold the pump housing in place. slide the impeller up over the drive shaft. Off with the old base plate, on with the new. Slide the new impeller down over the drive shaft, putting the woodruff key in the keyway just before the impeller covers it. Put a new housing gasket in place. Put a little grease in the new pump housing and slide it over the drive shaft. Turn the shaft clockwise while pushing the housing down over the impeller. Put the water tube and water tube grommet in place. Put a bit of molly lube on the drive shaft splines (but not on top of the driveshaft), so you won't have to pound it with a hammer to get it off next time. Use antisieze or something similar (I use OMC gasket sealant) on the lower unit retaining bolts to prevent corrosion from setting in and freezing them up. Most outboards call for something on the joint between lower unit and exhaust housing, I use 3m 847 but permatex #2 is ok too, just a little harder to clean next time around. Button it up and go fishing!
With a good service manual and a set of hand tools, you can do it, but there's too many details and to many variations. Don't rely on my memory - get the manual.How do I change the water pump in a 1978 40hp Mercury Outboard?This question is repeated in Boating questions almost daily. If your reasonably competent, you should be able to get through it with out much difficulty. First, get a service manual and read it before you go to unbolting. Second, get the parts kit, so you know what goes where. Then take your time and don't force anything. The actual procedure isn't much different from model to model.How do I change the water pump in a 1978 40hp Mercury Outboard?drop the lower unit and replace the pump.
