Hello, Garrett here with Boats.net and today I'm going to talk about the lower
unit on your outboard engine. Specifically, I'll use a cutaway model to
show you what it does and how it works. If you're looking for genuine
replacement parts for your lower unit or you're looking to buy a whole new lower
unit assembly, you'll find what you need in the lower casing one diagram on Boats.net.
In a nutshell, the lower unit, to coin an automotive terms, is where the rubber
meets the road. It's where the power and rotation of the
engine is transferred to the propeller to push your boat across the water. But
there's a lot more than meets the eye, so let's head over to the stand and I
can show you how it works. First thing we're going to talk about is the drive
shaft. This is what actually propels the forward and reverse gears that follow
with the prop shaft. You look towards the top, there's, you know, it's splined
around the top. This is what seats into the power head. As we work our way down
the drive shaft, we make contact with the water pump. The water pump is a very
important part to your outboard engine. It's what keeps everything cool. If it
fails it's going to lead to overheating and severe engine damage and nobody
wants that. I made a cutout for the lower unit, I also made a cutaway of water pump to
give you an idea exactly how it all goes together.
First thing we'll talk about is the lower plate. This is what everything
rests on top of. And above that is our actual impeller. This is what pushes the
water through the system, and it's connected to, of course,
the drive shaft. As I turn the drop shaft you can see the impeller begin to spin
inside the housing. Inside of this lower unit there's a cavity that allows water
to enter up into the bottom of the water pump housing. It enters from here, fills this
channel up, and floods the inside chamber of the water pump housing. As your drive
shaft spins, these impeller fins spin as well. And with this being full of water,
what's it going to do it's going to push the water around inside the housing and
exit out of this tube. Once it leaves the tube, it flows through your engine
cooling it properly and preventing you from having any overheating or, you know,
catastrophic engine failure. Nobody wants that.
Now this is a maintenance item which means it will wear out eventually.
So Yamaha recommends to check and replace every 100 hours or once per year.
Now I'll run mine quite often, so I change it way every 100 hours. You
definitely want to keep an eye on this and replace it because it could be out
on a Sunday afternoon with your family having a good grand old time and boom,
all of a sudden your engines overheating. Or you could be out in the ocean, 5 miles
out, and it'll start overheating. That's not good because you're no longer at the
top of the food chain at that point. And directly in front of the water pump
housing, we have the shift shaft. This is what actually allows us to shift from
forward to reverse gear. Ok, right now we're in the neutral position as we move
as we twist the shaft counterclockwise, it puts our engine in forward gear. And
as a shaft turns clockwise it shifts us into reverse gear. Now we're down here at
the bottom of the gearbox, so let me show you exactly what all is inside. We have a
race bearing, followed by your forward gear, your clutch dog-- that's where your
shift shaft engages to shift you from forward reverse gear. Your reverse gear,
and this whole section here is called the bearing carrier, and there's a couple
bearings. There's a bearing in the front, and the bearing towards the rear. That's
what helps distribute the load on the prop shaft. Behind the bearing carrier, we
have the housing ring. This is kind of what clamps everything back together. And
lastly, we have our prop. Now the next thing I want to talk about is our
forward, reverse, and pinion gear. Now if you look, if you notice as the drive
shaft spins, the forward and reverse gear spin along with the pinion gear. They're
constantly in motion whenever the engine is idling or at high or at max rpm. It's
constantly spinning. They're always engaged with each other. Now that you
pretty much know the basics of how the forward and reverse gears interact with
the pinion gear, let me show you exactly how it shifts from forward to reverse
gear. In between our forward and reverse gear, we have our clutch dog. Our clutch
dog is connected to our shift shaft. And as we turn our shift shaft left and
right, our clutch dog moves accordingly. And on either side of the clutch dog,
that has six fingers that actually line up with the forward gear and six fingers
that light up with the reverse gear. As this slides left to right, they engage.
And when they do that it allow us to drive that particular gear connected to
the prop shaft, which in our case is forward and reverse. I'm going to show you
as we turn our shift shaft to the forward gear position, our fingers are
engaged on the forward gear. When we shift in reverse, our clutch dog slides
and engages with the fingers on the reverse gear. Now I'm going to show you
exactly how this all works in motion. I'm gonna go ahead and select it into the
forward gear, spin our drive shaft. You can tell our clutch dog is spinning in
the forward motion. Now, we'll shift it in the reverse. You can spin our drive
shaft and you'll notice the clutch dog is spinning in the opposite direction.
Now if you ever hear any grinding going on your gearbox as you shift gears,
nine times out of ten that's gonna be your culprit. What happens is the fingers
on either side of the clutch dog, over time they begin to wear and round out.
And once that happens, you're actually bouncing in and out of gear. And to
prevent that from happening prematurely, always shift your engine at the lowest
rpm possible, which also means you need to make sure your engine idle is set
correctly on top of your shift in throttle cables. Because your shift and
throttle cables over time, they stretch. Everything wears, that's inevitable. We
can't do anything about that. But we can do is make sure it's all maintained
properly. Because what happens is if your shift cable is a little longer or
shorter than your throttle cable, your engine is actually gonna idle up just a
little bit before the shift engages. And when that happens you're spinning faster
than you need to be. The next thing I want to talk about are shims. I know
you've probably heard about it. Well let me tell you exactly what they do. The
whole purpose of a shim is to actually push out our gear faces on the forward
and reverse gears as well as the pinion gear to give us a good mesh between the
gears on our forward reverse and our pinion gear.
The shims go between each of the gears and they push the gears out. As you can
tell, I don't have any shims in this assembly at the moment because I want to
show you the importance of the shims. As you look, you can tell I can actually
wiggle this reverse gear back and forth. That's not what we want. We want it to be
rock solid, because we want the the mating surfaces of the pinion and the
gear to have little to no movement at all. What that's going to do is going to
be able to transfer power more efficiently and cause less wear. Now with
all these moving components inside the gearbox, of course we're gonna need some
kind of lubrication. This entire section is constantly submerged in oil and we
don't want any of that oil escaping or water getting into the system.
That's where seals come into play. On your outboard engine, the lower unit,
there's actually six points that keep water from getting in and oil escaping.
Let's go over those. First seals we'll talk about are the prop shaft seals.
These seals keep water in from entering and oil escaping at the prop shaft. But
they commonly go bad because this is a very exposed area for a seal, whether it
be fishing line get tangled in, sticks, or just regular debris. It's constantly
spinning on this seal face, so they do tend to go out. On the bearing carrier, we
have two o-rings. These are seal points as well. Inside of
this chamber is actually where your exhaust exits out the back of the
outboard. It flows down from power head, into this chamber, and directly out
through the prop. Our next spot is directly below the lower plate
underneath the water pump. Right there. And our last sealing point is around our
shift shaft. Have one-- we have a seal directly around the shift shaft and
there's an o-ring that runs around the outer perimeter of this housing. And the
best way to keep these sealed points in tip-top shape it's just doing your
routine maintenance. Yamaha recommends to replace the gear lube every 100 hours,
and as you replace it you can get it a good idea of what's going on inside the
lower unit. Say when you pull the drain plug and water comes out. Automatically,
that tells me that we have a leak somewhere in the system and it's going
to be one of those six points that I described to you. That's an easy way to
keep it in check. For the parts and supplies you need to keep the lower unit
on your outboard working its best, remember to check out the lower casting
one diagram on Boats.net. Have any questions or comments? Leave them in the
section below and I'll do my best to answer them.
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thanks for watching and have a great day.