Water Pump Impeller Kit – Fits Yamaha 40–50HP 2-Stroke Outboards (Replaces 6H4-W0078-00-00, 18-3429)
Water Pump Impeller Kit – Fits Yamaha 40–50HP 2-Stroke Outboards (Replaces 6H4-W0078-00-00, 18-3429) is backordered and will ship as soon as it is back in stock.
Worry-free returns
Worry-free returns
Shop with confidence. If you’re not completely satisfied, eligible items may be returned within the return window, provided they are unused and in original packaging.
If the return is due to our error or a defective item, we’ll make it right.
Please contact us before returning any item.
Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
International delivery times may vary due to customs processing.
Tracking information will be provided once your order ships.
Water Pump Impeller Kit – Fits Yamaha 40–50HP 2-Stroke Outboards (R...
Water Pump Impeller Kit – Fits Yamaha 40–50HP 2-Stroke Outboards (Replaces 6H4-W0078-00-00, 18-3429) is backordered and will ship as soon as it is back in stock.
Product Details
This complete Water Pump Impeller Kit – Fits Yamaha 40–50HP 2-Stroke Outboards (Replaces 6H4-W0078-00-00) provides all necessary components for a full water pump service. It is specifically designed for Yamaha 40HP (1984–1994), 50HP standard models (1984–1987), Pro Series 50 (1987–1990), and P50 Series (1995–1996) outboards.
It serves as a direct interchange for Yamaha part numbers 6H4-W0078-00-00, 6H4-W0078-01-00, and 6H4-W0078-A0-00, as well as aftermarket equivalents like Sierra 18-3429 and SouthMarine 6H4-W0078-A0/6H4-W0078-00.
- Engineered for a direct OEM-replacement installation process.
- Features a durable, heat-resistant neoprene impeller for reliable cooling performance.
- Includes all required gaskets, stainless steel hardware, and a Woodruff key for a comprehensive overhaul.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket suppliers.
| Brand | Part Number |
|---|---|
| Yamaha | 6H4-W0078-00-00 |
| Yamaha | 6H4-W0078-01-00 |
| Yamaha | 6H4-W0078-A0-00 |
| Sierra | 18-3429 |
| SouthMarine | 6H4-W0078-A0 |
| SouthMarine | 6H4-W0078-00 |
Engine & Model Compatibility
Confirm your engine model and year in the table below.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | All models | 40 | 2-Stroke | 1984–1994 | 6H4-W0078-00-00 | |
| Yamaha | Standard models | 50 | 2-Stroke | 1984–1987 | 6H4-W0078-00-00 | |
| Yamaha | Pro Series 50 | 50 | 2-Stroke | 1987–1990 | 6H4-W0078-00-00 | |
| Yamaha | P50 Series | 50 | 2-Stroke | 1995–1996 | 6H4-W0078-00-00 |
Technical Specs
- Product Type
- Water Pump Impeller Kit
- Installation
- Direct OEM replacement
- Housing Material
- Metal/Plastic composite
- Impeller Material
- Neoprene (heat-resistant, flexible)
- Hardware Material
- Stainless steel
- Gaskets
- Non-asbestos fiber (OEM-compliant)
- Blade Count
- 6
- Warranty
- 30 days (retailer dependent)
What’s in the Box
- 1 × Water Pump Impeller (6H4-44352-00)
- 1 × Insert Cup (663-44322-00)
- 1 × Wear Plate (663-44323-00)
- 1 × Gasket (663-44315-A0)
- 1 × Gasket (663-44316-A0)
- 1 × Gasket (663-44324-A0)
- 1 × Grommet (663-44366-00)
- 2 × Bolts (97080-08045)
- 2 × Bolts (97080-08055)
- 1 × Woodruff Key (90280-03024)
Installation Notes
- Disconnect the battery and shift the outboard into neutral.
- Drain the lower unit gear oil completely to prevent spills.
- Remove the lower unit mounting bolts and carefully detach the lower unit from the engine.
- Locate and unbolt the water pump housing to access the impeller.
- Remove the old impeller, wear plate, insert cup, and gaskets. Note the rotational direction of the old impeller vanes for correct installation of the new one.
- Thoroughly clean all mating surfaces, removing any old gasket material or sealant.
- Install the new gaskets, wear plate, and cup. Apply a light coat of marine grease to the inside of the housing.
- Slide the new Woodruff key onto the driveshaft and then slide the new impeller on, turning the shaft clockwise to bend the vanes into position.
- Reinstall the pump housing, ensuring a proper seal. Tighten bolts to the manufacturer's specified torque.
- Re-attach the lower unit, reconnecting the shift shaft and water tube. Refill the lower unit with fresh gear oil before operation.
Troubleshooting & FAQ
How can I confirm this kit fits my specific engine?
Your model must be listed in the Engine & Model Compatibility table. Please check your engine's model, horsepower, and year range against the table. If your model is not listed, this kit may not fit.
What are the signs of a failing water pump impeller?
Common symptoms include the engine overheating at idle or low speeds, a weak or non-existent stream of water from the "tell-tale" outlet, and in severe cases, an engine alarm. A brittle or cracked impeller upon inspection also indicates it's time for replacement.
How often should I replace my outboard's impeller?
As a general maintenance rule, it is recommended to replace the impeller every 1-2 years or every 100-200 hours of use, whichever comes first. For boats operated in saltwater or sandy/silty conditions, annual replacement is strongly advised.
Why is my engine still overheating after installing the new kit?
If overheating persists, verify the impeller was installed with the vanes rotating in the correct direction (typically clockwise). Check for blockages in the cooling water passages, intake screens, or the tell-tale outlet. Ensure the thermostat is functioning correctly, as it could be stuck closed.
Is there a difference between OEM and this aftermarket kit?
This listing is an aftermarket replacement, not an original equipment manufacturer (OEM) part. It has été built to meet or exceed OEM-level specifications using comparable high-quality materials. Our parts are manufactured in an ISO 9002 certified facility and are batch-inspected to ensure consistent quality and performance.
What are the shipping speed and cost?
We ship worldwide quickly, with typical transit times of 5–15 days for most regions. Standard shipping is free for all orders. Expedited shipping options and their costs will be displayed at checkout if available for your location.
What is the warranty and return policy?
This kit is covered by a 1-year warranty against manufacturing defects under normal marine use. If you encounter any issues, please contact our support team with your order details. Returns are accepted within our standard window, provided the item is unused and in its original packaging. Please see our full policy page or reach out for detailed instructions.
Safety & Important Info
- Verify all images and specifications against your original part before ordering. If anything differs—even if your model appears on the compatibility list—do not purchase.
- This is an aftermarket replacement component, not an OEM part. Reference numbers are provided for interchange purposes only.
- Professional installation by a qualified marine mechanic is highly recommended for safety, performance, and to prevent damage to your engine.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and rigorously tested for demanding real-world conditions.
- Fast Global Fulfillment — We offer free standard delivery to most regions, with parts typically arriving in 5–15 days from our integrated logistics network.
- Manufacturer-Direct Value — By working directly with our manufacturing partners, we provide fair pricing without compromising on materials or quality control.
Payment & Security
Payment methods
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
