Water Pump Repair Kit – Fits Yamaha 2-Stroke 40-50HP (Replaces 6H4-W0078-00-00, Sierra 18-3429)
Water Pump Repair Kit – Fits Yamaha 2-Stroke 40-50HP (Replaces 6H4-W0078-00-00, Sierra 18-3429) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
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Water Pump Repair Kit – Fits Yamaha 2-Stroke 40-50HP (Replaces 6H4-...
Water Pump Repair Kit – Fits Yamaha 2-Stroke 40-50HP (Replaces 6H4-W0078-00-00, Sierra 18-3429) is backordered and will ship as soon as it is back in stock.
Product Details
This Water Pump Repair Kit – Fits Yamaha 2-Stroke 40-50HP provides the essential components to restore your outboard's cooling system performance. It is designed for specific Yamaha 40HP (1984–1994), 50HP (1984–1987), Pro Series 50 (1987–1990), and P50 (1995–1996) models.
The kit serves as a direct replacement for multiple OEM and aftermarket part numbers, including Yamaha 6H4-W0078-00-00, 6H4-W0078-01-00, 6H4-W0078-A0-00, and Sierra 18-3429.
- Engineered for direct-fit installation, matching OEM specifications.
- Compatible with marine 2-stroke outboard cooling systems.
- The water pump housing is not included with this base kit and must be purchased separately.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number | Notes |
|---|---|---|
| Yamaha | 6H4-W0078-00-00 | |
| Yamaha | 6H4-W0078-01-00 | |
| Yamaha | 6H4-W0078-A0-00 | |
| Sierra | 18-3429 | Base kit without housing. |
| Sierra | 18-3408 | References a kit that includes the housing. |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This kit is verified to fit the following Yamaha outboard models within the specified year ranges.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | Outboard | 40 | 2-Stroke | 1984–1994 | 6H4-W0078-00-00, 6H4-W0078-01-00, 6H4-W0078-A0-00 | |
| Yamaha | Outboard | 50 | 2-Stroke | 1984–1987 | 6H4-W0078-00-00, 6H4-W0078-01-00, 6H4-W0078-A0-00 | |
| Yamaha | Outboard Pro Series 50 | 50 | 2-Stroke | 1987–1990 | 6H4-W0078-00-00, 6H4-W0078-01-00, 6H4-W0078-A0-00 | |
| Yamaha | Outboard P50 | 50 | 2-Stroke | 1995–1996 | 6H4-W0078-00-00, 6H4-W0078-01-00, 6H4-W0078-A0-00 |
Technical Specs
- Product Type
- Water Pump Repair Kit
- Installation
- Direct OEM replacement
- Housing Material
- Not included in base kit (housing sold separately)
- Other Technical Attributes
- Compatible with marine 2-stroke outboard cooling systems
What’s in the Box
- 1 x Water Pump Impeller (6H4-44352-00)
- 1 x Woodruff Key (90280-03M03-00)
- 1 x Cartridge Outer Plate (663-44323-00)
- 1 x Insert Cup (663-44322-00)
- 1 x Gasket (663-44315-A0)
- 1 x Gasket (663-44316-A0)
- 1 x Gasket (663-44324-A0)
- 1 x Water Seal (93101-23070-00)
- 4 x Bolt (97095-08045-00)
Installation Notes
- Disconnect the spark plug leads and the battery for safety.
- Shift the outboard into neutral and remove the lower unit (gearcase) bolts.
- Carefully lower the gearcase to access the water pump assembly.
- Remove the bolts securing the water pump housing and lift it off. Note the orientation of all parts.
- Remove the old impeller, plate, gaskets, and seals. Thoroughly clean all surfaces of debris and old gasket material.
- Install the new insert cup, gaskets, and outer plate from the kit.
- Install the new Woodruff key and slide the new impeller onto the driveshaft. Turn the shaft clockwise to seat the impeller vanes correctly.
- Apply a light coat of marine grease to the inside of the housing and reinstall it.
- Tighten the housing bolts to the manufacturer's recommended torque specification.
- Reinstall the lower unit, ensuring the driveshaft, shift shaft, and water tube are properly aligned.
- Once fully assembled, test the engine in water to verify proper cooling system operation.
Troubleshooting & FAQ
How do I confirm this kit fits my Yamaha engine?
The most reliable method is to cross-reference your engine's model and year against the "Engine & Model Compatibility" table. Additionally, you can check the part number stamped on your existing water pump components against the numbers listed in our "Part Number Matrix."
What are the signs of a failing water pump?
Common symptoms include a weak or intermittent "telltale" water stream, engine overheating alarms, or a noticeable decrease in engine performance. A brittle, cracked, or deformed impeller is a clear sign that replacement is needed.
How often should the water pump impeller be replaced?
As a general maintenance guideline, the impeller should be inspected annually and replaced every 1-2 years or every 100-200 hours of operation, whichever comes first. This interval may be shorter if you frequently operate in sandy or silty water.
Does this kit include the water pump housing?
No, this is a base repair kit that does not include the metal or plastic housing. If your housing is cracked, warped, or heavily scored, it must be purchased separately. This kit's cross-reference (Sierra 18-3429) is for a kit without the housing.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.
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