# Parts of an Outboard Motor Explained: Powerhead to Lower Unit > The parts of an outboard motor fall into four sections: powerhead, midsection, transom bracket and lower unit. Learn what each part does and which ones wear. Watercraft & Outboards By [MARS Engineering & Sales Team](/about/) Published Oct 23, 20267 min read The parts of an outboard motor fall into four main sections: the powerhead (the engine itself, under the cowling), the midsection (the driveshaft and exhaust housing), the transom bracket with its steering and tilt-and-trim gear, and the lower unit (the gearcase that turns the propeller). Power flows from a vertical crankshaft down the driveshaft to a pair of bevel gears in the lower unit, which turn the propeller shaft at a right angle. ## Key takeaways - An outboard motor is a self-contained engine, transmission and propeller unit that clamps or bolts to the boat's transom and steers by pivoting as a whole. - The powerhead contains the cylinder block, vertical crankshaft, fuel, ignition and cooling systems; four-strokes also have valves, camshafts and an oil sump. - The midsection carries the driveshaft, the cooling water tube and the exhaust passage, and isolates vibration through rubber mounts. - The lower unit houses the pinion, forward and reverse gears, the shift clutch, the propeller shaft and, on most outboards, the water pump. - Cooling water enters through intakes on the gearcase and is pumped up to the powerhead by a rubber impeller, which is a routine wear part. - The most frequently replaced outboard parts are the impeller, gear oil and seals, spark plugs, filters, anodes, thermostats and propellers. ## What is an outboard motor? An outboard motor is a portable or bolt-on marine propulsion unit that combines engine, gearbox, propeller and steering in one assembly mounted outside the hull on the transom. Unlike an inboard engine, it needs no shaft through the hull and no separate rudder, because the whole motor pivots to steer. Outboards range from about 2 hp portable tiller motors to production V8 and V12 engines of several hundred horsepower. Almost all share the same basic layout, so knowing the parts of an outboard motor helps when reading a parts diagram, ordering spares or diagnosing a fault. For how outboards compare with inboards and sterndrives, see [inboard vs outboard vs sterndrive](/blog/inboard-vs-outboard-vs-sterndrive/) . ## How does an outboard motor work? An outboard works by turning the engine's vertical crankshaft, sending that rotation down a long driveshaft, and converting it to horizontal rotation at the propeller through bevel gears in the lower unit. The gears also reduce speed, typically by a ratio of around 2:1, so the propeller turns slower than the engine and with more torque. - The powerhead burns fuel and turns the crankshaft, which sits vertically. - The crankshaft drives the driveshaft, which runs down through the midsection. - At the bottom, a pinion gear on the driveshaft meshes with forward and reverse bevel gears. - A sliding clutch dog, moved by the shift linkage, locks either gear to the propeller shaft, or neither for neutral. - The propeller shaft turns the propeller, which produces thrust. - The driveshaft also spins the water pump, which pushes cooling water up to the engine. - On most mid-size and larger outboards, exhaust gas exits through the propeller hub, below the waterline. ## What are the main parts of an outboard motor? The main parts of an outboard motor are the four core sections plus the cowling and propeller. Reading from top to bottom, any outboard motor diagram or parts catalog is organized into the sections below. | Section | Main parts | What it does | | --- | --- | --- | | Cowling | Top cowl, bottom cowl, air intake, latches | Protects the powerhead and guides intake air | | Powerhead | Cylinder block, crankshaft, pistons, cylinder head, fuel and ignition systems, flywheel, starter | Produces power | | Midsection | Driveshaft housing, driveshaft, water tube, exhaust passage, mounts, oil sump (four-strokes) | Connects powerhead to lower unit and isolates vibration | | Transom bracket | Clamp bracket, swivel bracket, tilt tube, steering arm or tiller, tilt and trim unit | Mounts, steers, tilts and trims the motor | | Lower unit (gearcase) | Pinion, forward and reverse gears, clutch dog, propeller shaft, water pump, seals, bearings | Transmits power to the propeller and supplies cooling water | | Propeller | Blades, hub, shock insert, thrust washer, nut | Converts rotation into thrust | ## What are the parts of the powerhead? The powerhead is the engine block and everything bolted to it. On a four-stroke, it is essentially a car-type engine turned upright so the crankshaft is vertical. - **Cylinder block, pistons and connecting rods:** convert combustion pressure into crankshaft rotation. - **Cylinder head, valves and camshafts (four-stroke):** control intake and exhaust, driven by a timing belt or chain. - **Reed valves and transfer ports (two-stroke):** replace the valve train in a two-stroke; see our [2-stroke vs 4-stroke outboard comparison](/blog/two-stroke-vs-four-stroke-outboard/) . - **Fuel system:** a carburetor on small and older motors, or electronic fuel injection with a lift pump, vapor separator, high-pressure pump, injectors and throttle body. - **Ignition and electrics:** engine control unit or ignition module, ignition coils, spark plugs, and a flywheel with charging coils that power the ignition and charge the battery. - **Starting:** a recoil (pull) starter on portable motors and an electric starter motor on most motors above about 25 hp. - **Cooling passages and thermostat:** water jackets around the cylinders, with a thermostat and pressure relief valve to control temperature. - **Lubrication (four-stroke):** oil pump, oil filter and oil pickup in the sump. ## What does the midsection do? The midsection, also called the driveshaft housing or exhaust housing, connects the powerhead to the lower unit and sets the motor's shaft length. Its length is what distinguishes a short, long or extra-long shaft motor, as explained in our [outboard shaft length guide](/blog/outboard-shaft-length-guide/) . Inside run the driveshaft, the water tube that carries cooling water up from the pump, the shift rod or cable, and the exhaust passage that leads gas down toward the propeller. On four-strokes, the oil sump usually sits at the top of the midsection under the powerhead. Rubber mounts between the midsection and the swivel bracket absorb engine vibration so it is not transmitted to the hull. ## What are the transom bracket and tilt-and-trim parts? The transom bracket is how the motor attaches to the boat. The **clamp bracket** bolts or screws to the transom, and the **swivel bracket** pivots inside it on a horizontal **tilt tube**, so the motor can tilt up and out of the water. Steering is through a **tiller arm** on small motors or a **steering arm** linked to cable or hydraulic steering on larger motors. On motors from roughly 30 to 40 hp upward, a **power trim and tilt unit** with an electric motor, hydraulic pump and rams adjusts the running angle under way and lifts the motor for trailering or shallow water. Smaller motors use manual tilt with a tilt lock and a trim pin that sets the running angle. ## What is in the outboard lower unit? The lower unit, or gearcase, is the outboard's transmission. It contains the pinion gear, the forward and reverse gears, the clutch dog that selects them, the propeller shaft with its bearings and seals, and gear oil that lubricates all of it. On most outboards, the **water pump** sits on top of the gearcase, driven by the driveshaft. Its flexible rubber impeller draws water through intake screens on the gearcase and pushes it up the water tube. The lower unit also carries the **anti-ventilation plate**, the **skeg** that protects the propeller, and often a **trim tab** that doubles as a sacrificial anode to protect the aluminum against galvanic corrosion. The most common lower unit problem is water intrusion. If drained gear oil looks milky, water has entered past a seal, and the propeller shaft or driveshaft seals should be replaced before the gears and bearings are damaged. Fishing line wound behind the propeller is a frequent cause of seal failure. ## Which outboard parts wear out and how often? Most outboard wear parts are replaced on an annual or 100-hour cycle, though intervals vary by model and duty, so follow the owner's manual. | Part | Typical interval | Why it matters | | --- | --- | --- | | Engine oil and filter (four-stroke) | 100 hours or yearly | Protects bearings, cams and valves | | Gear oil | 100 hours or yearly | Water in gear oil destroys gears and bearings | | Water pump impeller | Inspect yearly; replace per manual | A worn impeller causes overheating | | Spark plugs | 100 hours or yearly | Hard starting and misfire | | Fuel filters and water separator | 100 hours or yearly | Water and dirt damage injectors and carburetors | | Anodes | Check yearly; replace when about half eroded | Protect the gearcase and midsection from corrosion | | Thermostat | Inspect per manual | Overheating or overcooling | | Propeller | Inspect every trip | Dings and bends cause vibration and lost speed | For fleet schedules, see the [outboard and PWC fleet maintenance schedule](/blog/outboard-and-pwc-fleet-maintenance-schedule/) and the step-by-step [outboard maintenance and winterizing guide](/blog/outboard-motor-maintenance-winterizing/) . ## How do I order the right outboard parts? Order by model code and serial number, not by horsepower alone. The same horsepower can be built on different powerheads, gearcases and model years, and parts often change at a serial-number break. The serial plate is usually on the swivel or clamp bracket. When asking a supplier for parts or a complete motor, give the full model code (it usually includes shaft length and starting type), the serial number, the year, photos of the plate and the part, and the part number from the parts diagram if you have it. MARS supplies complete [outboard motors](/products/outboard-motors/) and [engine parts](/products/engine-parts/) for export; send the details through the [contact page](/contact/) for a quotation. ## Frequently asked questions What is the lower unit on a boat motor? The lower unit is the gearcase at the bottom of an outboard or sterndrive. It contains the gears, clutch and propeller shaft, and on outboards usually the water pump. Where is the water pump on an outboard motor? On most outboards the water pump sits on top of the lower unit gearcase, driven by the driveshaft. The lower unit must be removed to replace the impeller. What is the difference between the powerhead and the lower unit? The powerhead is the engine that produces power, while the lower unit is the gearbox that transmits that power to the propeller. The midsection connects the two. What does milky gear oil in an outboard mean? Milky gear oil means water has mixed with the oil, usually through a worn propeller-shaft or driveshaft seal. Replace the seals and refill with fresh gear oil before further use. What is an inboard-outboard motor? An inboard-outboard, or sterndrive, has the engine inside the hull and an outboard-style drive leg on the transom. It shares the lower-unit concept with an outboard but is a different propulsion system. M **MARS Engineering & Sales Team** Written by the engineering and sales team at Shanghai Mars Industrial Co., Ltd., who specify, inspect and ship marine and industrial power equipment from Shanghai every week. ## Share - [Share on LinkedIn](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Foutboard-motor-parts-explained%2F) - [Share on WhatsApp](https://wa.me/?text=Parts+of+an+Outboard+Motor+Explained%3A+Powerhead+to+Lower+Unit+https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Foutboard-motor-parts-explained%2F) - [Share by email](mailto:?subject=Parts+of+an+Outboard+Motor+Explained%3A+Powerhead+to+Lower+Unit&body=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Foutboard-motor-parts-explained%2F) **Products mentioned** - [Mercury 115 ELPT FourStroke Outboard Motor (115 HP)](/products/outboard-motors/mercury-115-elpt-fourstroke-outboard-motor-115-hp/) - [Mercury 60 EFI FourStroke Outboard Motor (60 HP)](/products/outboard-motors/mercury-60-efi-fourstroke-outboard-motor-60-hp/) - [DF90A 4-Stroke Inline-4 Outboard Motor](/products/outboard-motors/df90a-4-stroke-inline-4-outboard-motor/) - [DF9.9BT EFI 4-Stroke Portable Outboard Motor](/products/outboard-motors/df9-9bt-efi-4-stroke-portable-outboard-motor/) - [Mercury 40 EFI FourStroke Outboard Motor (40 HP)](/products/outboard-motors/mercury-40-efi-fourstroke-outboard-motor-40-hp/) ## Need a quotation? Send your model, application and quantity — we reply within 24 hours on working days. 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