The Parts of an Outboard Motor — Complete Breakdown Guide
If you own an outboard motor or plan to buy one, you must understand how the various components work so you can maintain the engine and increase its life span. We have prepared this comprehensive guide to help you diagnose and repair the different parts of your outboard motor.
What Is an Outboard Motor?
An outboard motor is an external engine designed to propel watercraft. The engine, propeller and gearbox are in one combined unit installed on the transom, creating more space inside the boat. Outboard motors are popular because they have a high horsepower-to-weight ratio and extended maintenance intervals.
There are different types of outboard motors, depending on application and performance. Two common examples are the two- and four-stroke outboard engines.
- Two-stroke outboard motors: These are the simplest of the two. Their pistons complete a combustion cycle in two movements and do not require a valve train. Two-stroke engines are typically smaller and lighter but run faster.
- Four-stroke outboard motors: These complete a combustion cycle in four movements. They are gasoline- or diesel-powered, with an oil filter and a self-contained oil cavity within the engine. Four-stroke outboard motors are ideal for spending long hours on the water.
Outboard motors are high-powered engines with multiple parts, each performing a specific function. You must know what these components are and what they do to help you repair and maintain them. Most engines have original equipment manufacturer and aftermarket parts, which vary in pricing, fitting, quality and warranty. It’s critical to find a reliable supplier with components that give you maximum value at a reasonable price.
Parts of an Outboard Motor
Outboard motors have three primary sections — the outboard powerhead, midsection and outboard lower unit.
The powerhead sits outside the water and comprises components that make the engine run, such as the ignition and timing, lubrication, transmission, fuel, electrical and cooling systems.
1. Ignition and Timing System
The ignition and timing system starts and stops the engine by producing an electrical spark to ignite the fuel/air mixture in the cylinders. For two-stroke outboard motors, the blend also contains a small amount of oil to lubricate the engine. The ignition and timing system has many parts, including the following.
- Spark plug: The plug ignites the mixture of fuel and air within the cylinders to generate power to propel the boat. Connecting the spark plug to the ignition coil is a wire that carries a high-voltage current. These components consistently experience harsh conditions during combustion, causing them to wear out over time.
- Camshaft: By opening and closing the valve, the camshaft allows the fuel/air mixture to enter and exit the combustion chambers. The camshaft operates the exhaust valves in two-stroke engines. However, in four-stroke engines, it controls the intake too.
- Crankshaft: As the internal combustion engine's backbone, the crankshaft connects to pistons inside the engine block. The pistons move up and down, causing the shaft to rotate and generate power.
- Timing gears: The timing gears rotate, transmitting the crankshaft's movement to the camshaft to open and close the intake and exhaust valves. A chain or belt connects the timing gear to the crankshaft at one end and the camshaft at the other. Most modern engines use timing belts or chains to synchronize crankshaft and camshaft rotation, though some use the gears to drive the camshaft directly.
- Timing belt or chain: Some outboard motors have timing chains, while others have belts. The timing belt or chain times the valve's opening and closing with the piston's motion for smooth operation.
- Tensioners and idler pulleys: Tensioners prevent slackness in the timing belt, while idlers route the belt around the ancillary parts. Tensioners also provide pressure on the belt to help the pulleys move correctly. On the other hand, idlers regulate the belt attached to the crankshaft to generate movement.
The oil filter is the primary lubrication system in the outboard motor. It removes contaminants from the engine oil, enhancing performance and increasing the motor’s life span. The oil filter has a metallic can on the outside with a sealing gasket, allowing it to hold tightly against the engine's mating surface. The can’s base plate secures the perforated gasket with a screw threaded underneath so it fits on the oil filter assembly on the engine block.
The can has a filter material, typically made from synthetic fiber. The oil pump forces the oil into the filter through the holes, pushing the dirty oil under pressure through the fiber and back through the threaded central hole, where it re-enters the engine.
The outboard motor transmission system controls the engine power or propulsion. It has several interconnected parts.
- Gears: These send power from the engine to the propeller, converting the motor's high-speed rotational motion into the required speed and torque to drive the propeller.
- Clutch: A dog clutch sits between the forward and reverse gears. When the driver pushes the shift shaft forward, it engages the clutch with the forward gear and causes it to rotate. When the clutch is in reverse, it engages it with the reverse gear and causes it to turn in the opposite direction.
- Torque converter: This component transmits torque from the engine to a rotating driven load. Torque represents the power in the machine or the amount of work it can do.
4. Fuel System
The fuel system delivers fuel to the engine for power. There are different parts, but we can categorize them into three.
Fuel Tank and Lines
The fuel tank and lines store and release fuel to the other parts of the outboard engine with three primary components.
- Vent lines: Hoses and tubes used to maintain pressure in the fuel tank.
- Supply lines: Hoses or pipes that connect the fuel tank to the engine.
- Hoses and connectors: Fuel hoses transport fuel from the tank to the engine. Connectors allow for easy installation of outboard motor fuel lines.
Below are the vital fuel delivery components.
- Carburetor: Outboard engines need the proper amount of oxygen to burn the fuel, and that's where the carburetor shines. The carburetor mixes air and fuel for efficient combustion. It has different parts, including the air cleaner, choke, needle valves, throttle cable and float.
- Fuel injectors: Like carburetors, fuel injectors mix fuel and air and inject them into the internal combustion engine. However, injectors are usually features on newer boats. Fuel injectors are efficient and provide better fuel economy.
- Fuel pump: An outboard motor’s fuel pump does not operate with a mechanical source like inboard engines. A diaphragm-type fuel pump mounted on the powerhead draws fuel from the fuel system and delivers it to the injector or carburetor.
- Fuel pressure regulator: As the name suggests, the fuel pressure regulator regulates the fuel pressure that goes into the injectors.
- Engine control unit: The engine control unit or module controls multiple systems, including fuel injection and ignition.
Primary and secondary fuel filters sit between the fuel tank and the engine, where they remove impurities from the fuel before it reaches the outboard motor. Primary filters are often near the fuel tank and are responsible for removing large amounts of contaminants. Secondary filters are near the engine. They remove smaller contaminants.
5. Electrical System
The electrical system stores, starts and distributes electrical power to the rest of the engine. It has many components, but the power supply and ignition are the most critical.
The following two components are responsible for power supply.
- Battery: Like any other battery, it is a power source for the outboard motor. Read your owner’s or service manual to know the right type and size of battery to use.
- Starter motor: The starter has an electric motor that turns over the engine to start. It has a powerful direct current electric motor and starter solenoid — a relay between the battery and the starter motor.
The ignition system's electrical parts include the following.
- Ignition coil: Transforms the voltage from the ignition pack into a spark voltage. The spark plugs use the voltage to ignite the fuel.
- Stator: Provides power to the entire ignition system and the battery charging circuit.
- Ignition control module and ignition timing: The ignition control module regulates the ignition timing and generates sparks to ignite the mixture of fuel and air in the cylinders.
- Wiring and fuses: A series of wires that connect the electrical components throughout the vessel. The cables are color-coded for easy identification. Fuses act as a safety valve to prevent current from exceeding the rating of wiring and electrical circuit components.
6. Cooling Systems
The outboard engine cooling system parts include those that regulate water flow and circulation.
Water Flow Regulation
Here are the four vital parts responsible for water flow regulation.
- Water inlets: Direct water intake into the cooling system.
- Screens and grates: Prevent foreign objects like grass, sticks and small stones from entering the cooling system and potentially damaging it.
- Hoses: Facilitate the flow of coolant — typically water — between different components to ensure the engine operates at optimal temperature.
- Thermostat: Prevents the cooling water from getting to the cylinder walls until there is adequate heat for efficient combustion. When the temperature in the cylinder rises, the thermostat allows cooling water to flow from the water pump through the water jacket and out the exhaust.
The water pump is the heart of the system, consisting of the following parts.
- Impeller: They have curved blades, allowing them to draw water from the surroundings and circulate it through the engine’s cooling system. Impellers can cause the engine to overheat when they break or get damaged.
- Housing: Encloses and supports the impeller, creating room for efficient water circulation.
- Seals and gaskets: Prevent leaks in the cooling system. Several locations have seals, including the shaft and water plate. Gaskets are flexible or flat materials between two mating surfaces for a tight seal.
- Wear Plate: Designed to guide and protect the impeller, ensuring efficient water circulation.
Outboard Midsection — What Is It?
The midsection connects the engine with the lower unit. It has many parts, though not as many as the powerhead, including the following.
1. Exhaust System
The exhaust system discharges the gases produced during combustion. It also helps reduce noise and expel heat. Components like these make up the system.
- Exhaust manifolds: Collect extremely hot exhaust gases directly from the engine and expel them through the exhaust pipe.
- Exhaust pipes: Collect exhaust gases from the manifolds and send them outside the machine. These pipes are usually near the powerhead on the side or back of the outboard motor.
- Silencers and mufflers: As the names suggest, silencers and mufflers reduce noise pollution from the exhaust system.
- Exhaust gaskets: Exhaust gaskets' primary function is to prevent leaking and ensure the exhaust gas flows through the catalytic converter between the engine head and the manifold.
2. Control System
The control system regulates the boat's speed, direction and overall performance. It consists of several parts.
- Throttles: Increase and decrease the boat's speed.
- Shift control: Changes the gears on the engine, such as forward, reverse and neutral.
- Steering system: Turns the boat in the preferred direction. There are three types — the tiller, mechanical and hydraulic steering.
- Trim and tilt control: Adjust the outboard engine by lowering or raising it. The engine's angle affects the boat's maneuverability and speed.
- Kill switch: A safety device designed to shut off the outboard motor when the operator is not at the helm.
3. Mounting System
The system used to mount the outboard motor onto the boat comprises the following parts.
- Mounting brackets: Secure the outboard motor on the boat’s transom and provide support, stability and adjustability.
- Mount bolts: Fasten the outboard motor to the mounting bracket and the boat’s transom.
- Transom pad: Protective pads or plates installed on the boat’s transom prevent the clamp inserts from causing dings, scratches and excessive wear, especially in cases involving high-powered or heavy outboard motors.
Outboard Lower Unit — What Is It?
Like the powerhead, the outboard motor's lower unit has many moving parts. This section transfers the power and rotation from the engine to the propeller and prop shaft. Below are the vital parts.
1. Gearbox Components
The gearbox components consist of bearings and shafts.
- Driveshaft: Takes the rotational power from the powerhead and transmits it to the water pump and gearcase.
- Propeller shaft: Connects the engine to the propeller and transmits rotational energy to move the boat.
- Bearings: The prop shaft, forward gear and reverse gear mount on the bearing for support. The bearing also allows free rotation within the gearcase.
2. Seals and Gaskets
Seals and gaskets help keep the lubricating oil within the gearbox and prevent water from entering the gearcase. You can locate oil seals and gaskets around components like the prop shaft, driveshaft, shift shaft, shift shaft plate and gearbox bearing housing.
Propellers transfer the engine horsepower to the water, causing it to move. They consist of the following components.
- Blades: Propeller blades have a forward-facing side called the pressure face, and an aft-facing side called the suction face. The pressure face pushes the water away from the front through thrust, while the suction face pulls water into the propeller. The blades also have leading and trailing edges.
- Hub: The propeller hub is the center-round portion attached to the blade.
- Diffuser ring: Prevents exhaust gases from feeding back into the blades and lowering the thrust.
- Keyway: A machine slot or groove in the propeller's hub that allows the shaft and propeller to mate.
- Straight ribs: The connections between the inner and outer hub on through-hub exhaust propellers.
- Inner hub: The sacrificial composite center between the hub and the splines that protects the drivetrain when the propeller strikes a hard object.
The skeg is a fin-link extension located underneath the outboard motor's lower unit. It helps protect the propeller, maintain the direction, stabilize the ride and steer the boat. You may find the following components on some outboard engines.
- Ventilation plates: The ventilation or anti-cavitation plate prevents the propeller from ventilating or sucking in air, reducing cavitation and ensuring consistent water flow.
- Cavitation plates: The cavitation plate or trailing edge plate is usually on the rear lower end of the skep, below the propeller. It helps redirect water flow.
You should also know about these outboard motor accessories.
You can get three essential covers to protect your outboard motor.
- Full covers: Shield the entire outboard engine when the boat is out of the water.
- Cowling cover: Removal covers used to safeguard the uppermost portion of the outboard motor.
- Propeller bag: Protects the propellers, especially during transportation.
2. Maintenance and Security
You can use different equipment to maintain and secure your outboard engine.
- Trolleys, stands and lifts: Used to safely lift, transport and store the outboard motor.
- Outboard motor locks: Some thieves target expensive boat engines. You can lock yours to prevent theft.
Get Outboard Motor Replacement Parts From Fawcett Boat Supplies
Fawcett Boat Supplies has provided boaters with high-quality components since 1948. Our extensive product selection makes us the country's best go-to marine supply store. Do you need help finding a replacement part or servicing your outboard motor? Browse our selection of OEM and aftermarket parts online or contact our store to speak with an expert today!