What is an Outboard Powerhead?


As a Marine Technician I get to talk to a lot of boat owners and users. Some know a lot about their boat and engine, and others not so much. Some have spent years on the water and others can barely get the boat in gear! Talking with so many people there is a question that a lot of people ask about.

What is an Outboard Power head? The Powerhead is the collection of the main components that make up a combustion engine. It is the block, pistons, heads, and valves all together.

All of these components put together is what makes up the engine. Of course there are other things like the fuel system, the electrical components, the midsection, and the lower unit. The powerhead though, is just the bare engine without the stuff that makes it work. Here is a break down of what makes up the powerhead.

A complete outboard powerhead

The Powerhead Begins With the Block

The inside of an outboard powerhead

The engine block is the beginning to the making of an outboard powerhead. The engine block is the part of the engine that houses the major moving components in the engine. Such as the crankshaft, piston rods, and pistons themselves.

The engine block is the absolute basic portion of the engine and basically creates the engine crankcase. This is where all of the oil is moving around all of the components that are moving inside of the engine.

The other components to the block are the actual cylinders. The cylinders of the engine are where the pistons are and is where all of the action happens.

The actual strokes of the engine happen inside of the block. For 2-strokes, there is intake and exhaust strokes. Then for 4-strokes you have the intake, compression, combustion, and exhaust strokes.

The Crankshaft & Pistons

The crankshaft is what the pistons attach to inside of the engine block. The crank spins around moving the pistons up and down inside of the cylinders creating the power of the engine.

The crank of a powerhead

The crank houses the piston rods which have the bearings for the piston rods. The piston rods are what connects the actual piston to the crank. As the engine’s crankshaft spins around, the piston rods move up and down moving the pistons inside of the cylinders.

On top of the crankshaft is the flywheel. The flywheel is what the starter engages to get the engine spinning before the combustion portion can take over and keep the engine running.

Also, the flywheel often has belts that attach to it that can run miscellaneous components of the engine. Such as the alternator, a supercharger, cams, or even a water pump on inboards.

The flywheel can also house a stater.  The stater is another form of an alternator but does not have the same ability to create as much power as you can get out of an alternator. All of this completes what makes up the crankshaft and internal block components of the engine.

Then There are the Cylinder Heads.

On top of the cylinders themselves you have the cylinder head. This is the component that houses the camshafts, cams, the valves, and the spark plugs depending on whether you have a 4-stroke or a 2-stroke.

With a 2-stroke engine, the engine utilizes ports which allow the fuel, air, and oil mixture to come into the cylinder. Then after the spark plug is ignited the piston will go down. Utilizing exhaust ports which allows the next batch of fuel and air mixture to push the burned off fuel mixture out.

The top of an outboard powerhead

With a 4-stroke engine, the engine utilizes valves that open and close allowing air and fuel mixture to come in and the exhaust to go out. Inside of the cylinder head the engine has what’s called a camshaft. The camshaft works the same way the crankshaft works. Just opening and closing the valves.

The intake valve opens and fuel and air are sucked into the cylinder. The valve is then closed and the piston comes up to the top of the cylinder, compressing the mixture.

The spark plug is then ignited and explodes the mixture. Driving the piston back down making the power stroke. Then the exhaust valve is opened, so that when the piston comes back up, it pushes the burned mixture out the exhaust side of the cylinder head.

Then as the piston goes down, the intake valve is opened back up, starting the cycle over by sucking in a new mixture of air and fuel. The cylinder head is also where the spark plug is positioned.

The cylinder head is attached to the top of the cylinders which make up the engine block. These components together make up the main portion of the power head.

The Cooling Passages

The inside cylinders of a powerhead

Integrated into both the engine block, cylinders, and the cylinder heads are cooling passages. These cooling passages are channels that are connected between all of these components which allow cooling water to flow through the power head.

The water absorbs the heat being produced by the engine and cools it down, so that it does not heat up to a temperature which can melt the metals in the engine and lock it up.

Incorporated into the cooling passages is where the thermostat is located. The thermostats’ job is to regulate the temperature of the water flowing through the cooling passages inside of the power head.

If the engine runs to hot it can lock up. If the engine runs to cold, it can produce condensation and create water build up in the crankcase and in the oil. Thus, also giving the potential to lock up the engine.

It is crucial that it runs at a certain temperature to avoid either of these conditions, which is the job of the thermostat!

Is It Worth Replacing a Power head or Should You Repower?

A fully dressed powerhead

To be honest with you. Every single persons situation is going to be different. There are a lot of variables that weigh into the answer to that question. To make it brief. The type and year of the engine plays a big part in it.

If the engine is 20-30 years old and has lived it’s entire running life in saltwater. It is probably not worth replacing the power head.

If most of the components on the engine are questionable, rusting to pieces, and starting to fall apart. Don’t waste the money on the power head. That is unless you are a confident mechanic that does not mind getting in and tearing it down.

Furthermore, if you can find a decent used power head for the right price, then it is probably worth replacing.

On the other hand, if you are not so confident and are going to have to pay someone to replace it. That’s a no. Getting things apart without breaking something else, becomes a very real issue that can get expensive in only a couple of hours. Not to mention you are back to running a 20 year old engine.

Now if your motor is say 5-10 years old and still in good condition, but your warranty has just expired. Then I would say go for it. You will save yourself in the long run. That is depending on what caused the failure. If the power head jumped time and blew up. Replace it and get back on the water.

If there is another underlying issue. Resolve that first when weighing the cost effectiveness of replacing the power head vs. repowering the boat.

You should now have a pretty good understanding of what exactly an outboard power head is.

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Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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