Home › Forum › Ask A Member › Impeller – key vs pin to shaft › Re: Impeller – key vs pin to shaft
A 40hp (through 1972) has a dual tube, recirculating cooling system. None of the major castings nor pump are the same as a 33hp. In the 40hp, if the thermostat is closed, water returns to the pump via the second water tube. Then it is recirculated to the powerhead. This continues till the thermostat opens, and the water is discharged and replaced with fresh cool lake water. This is not an either/or situation. The thermostat constantly samples the water temp and opens or closes as necessary to maintain proper temperature.
A 33hp does not have a thermostat or water return system.
If you want to really get confused, 40hp Gale-built motors have the 33hp castings and cooling system. So do the 1973-up Johnson/Evinrudes.