Tools, & Craftsmanship
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& Spark Plugs
Motor Lower Unit
Motor Cooling System
- Take a water pump apart. Identify the three main parts. What
causes the impeller to turn as the shaft turns?
The drive shaft has an indentation that holds a key. The key
also fits into the impeller in the pump. As the shaft turns, the
water pump also turns.
- Touch the bottom plate of the water pump with a file. Is it
harder or softer than the file? Why do you think this is so?
It is hard. It must be durable to withstand the abrasion from
- There are two types of metal that water pumps are made of.
Find examples of each. Why do you think there is a difference?
Aluminum housings are cheaper, but less durable. Chrome housings
are very durable but more expensive.
- Put an impeller in a pump housing. With a stick in place of
the drive shaft, turn the impeller. Watch the impeller blades
extend and compress. Imagine the water coming in the side where
the blades are extending and leaving the side where it is compressing.
- Find the intake for the water on the lower unit. Is there a
screen of any kind? Why do you think the screen is there? Ask
people in the village what it is for and what would happen if
it were missing.
There is usually a screen to keep rocks and grass out. The
cooling system, particularly the thermostat would plug up if it
were not there.
- Check the copper tubing that carries the water from the pump
to the upper unit. Are there seals where it connects to each?
Yes, there is pressure in the cooling system from the pump,
and the seals hold the pressure.
- Can you find the thermostat? They are in different places on
different motors. If possible, remove it. Alternately, pour hot
and cold water on it. Can you see it move in response to the temperature
The response is visible.
- If you live near a silty river, collect some water and let
it settle overnight. Is there sediment on the bottom of the container?
Pour most of the water off and rub your hand on the bottom of
the container. Can you see why silt will wear the impeller inside
the pump housing?
- What would happen to an outboard if water didnt circulate
around the cylinders? Is this change permanent?
Parts would warp because of overheating. Yes
- What are the three parts of a water pump?
Impeller, housing, bottom plate
- With a drawing of the top view, show how a water pump works.
It should show the impeller off center.
- What is the purpose of the screen on the lower unit?
Keep rocks and grass out
- What is the purpose of a thermostat? How can it malfunction?
Control the waterflow so the engine can work at the best temperature.
If a rock gets stuck, it can refuse to open or close.
- What does the indicator do? If it is plugged, does that mean
the water pump isnt working?
The indicator shows if there is water coming out of the cooling
system of the engine. If the indicator is not showing water, the
water pump may or may not be working.
- Why should an outboard motor be tilted up when not in use?
During what season is this not so?
Silt settles in the waterpump if the engine isnt tilted
up. This isnt true in late fall when the water is no longer
- A water pump doesnt work. The operator improvises a way
to get home. What are the consequences if the improvised water
cooling system doesnt work?
If his improvised method doesnt work well, the engine might
still run, but parts will be warped and damaged from the heat.
- A cheap aluminum impeller housing costs $12. An expensive stainless
steel one costs $37. However, a housing made from stainless steel
lasts 4 times longer. Which is cheaper in the long run?
The stainless one.