Water Pump Repair Kit Without Housing for Yamaha 40–50 HP 2-Stroke Outboards, 1984–1994 – Replaces 6H4-W0078-00-00, 6H4-W0078-01-00, Sierra 18-3429
Water Pump Repair Kit Without Housing for Yamaha 40–50 HP 2-Stroke Outboards, 1984–1994 – Replaces 6H4-W0078-00-00, 6H4-W0078-01-00, Sierra 18-3429 is backordered and will ship as soon as it is back in stock.
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Water Pump Repair Kit Without Housing for Yamaha 40–50 HP 2-Stroke ...
Water Pump Repair Kit Without Housing for Yamaha 40–50 HP 2-Stroke Outboards, 1984–1994 – Replaces 6H4-W0078-00-00, 6H4-W0078-01-00, Sierra 18-3429 is backordered and will ship as soon as it is back in stock.
Product Details
This water pump repair kit without housing is designed to service the cooling system on selected Yamaha 40 HP and 50 HP 2-stroke outboards. It replaces the serviceable water-pump components while allowing you to reuse the existing water-pump housing when it is in good condition.
The kit is associated with Yamaha OEM references 6H4-W0078-00-00, 6H4-W0078-01-00, and 6H4-W0078-A0-00. It also cross-references to Sierra 18-3429, a water pump kit specified without the housing.
- Aftermarket replacement water pump repair kit for selected Yamaha 2-stroke outboards.
- Includes the impeller, key, insert cup, wear plate, gaskets, water seal, and replacement bolts listed below.
- Water-pump housing is not included. Inspect the original housing and replace it separately if it is cracked, warped, or heavily scored.
- Confirm the complete engine model and year before ordering; horsepower alone is not sufficient to establish fitment.
Part Number Matrix
Use these references to compare the kit with your existing service information and water-pump components.
| Brand | Part Number | Reference |
|---|---|---|
| Yamaha | 6H4-W0078-00-00 | Water pump repair kit reference used on selected 1984–1991 applications. |
| Yamaha | 6H4-W0078-01-00 | Water pump repair kit reference used on selected 1992–1994 applications. |
| Yamaha | 6H4-W0078-A0-00 | Cross-reference listed for the water pump kit without housing. |
| Sierra | 18-3429 | Water pump kit without housing. |
| Sierra | 18-3408 | Related water pump kit with housing; it is not the same package as this without-housing kit. |
Engine & Model Compatibility
Fitment depends on the exact Yamaha model code and year. The catalog relationships support the following application ranges for this water-pump kit family:
| Brand | Engine Family | Rated HP | Stroke | Years | Applicable Reference | Notes |
|---|---|---|---|---|---|---|
| Yamaha | Selected 40-series outboards | 40 | 2-Stroke | 1984–1991 | 6H4-W0078-00-00 | Verify the complete model code and lower-unit configuration. |
| Yamaha | Selected 40-series outboards | 40 | 2-Stroke | 1992–1994 | 6H4-W0078-01-00 | Verify the complete model code and lower-unit configuration. |
| Yamaha | Selected 50-series outboards | 50 | 2-Stroke | 1984–1991 | 6H4-W0078-00-00 | Includes selected Pro50 applications; confirm the model code. |
| Yamaha | Selected 50-series outboards | 50 | 2-Stroke | 1992–1994 | 6H4-W0078-01-00 | Confirm the complete model code before ordering. |
| Yamaha | Selected P50 outboards | 50 | 2-Stroke | 1992–1994 | 6H4-W0078-01-00 | Verify the complete P50 model code. |
Technical Specs
- Product Type
- Water pump repair kit without housing
- Application
- Selected Yamaha 40–50 HP 2-stroke outboards
- Installation
- Replacement service parts for the original water-pump assembly
- Housing
- Not included; reuse the original housing only after inspection
- Aftermarket Status
- Aftermarket replacement; not a Yamaha-manufactured OEM component
What’s in the Box
- 1 x Water pump impeller — Yamaha reference 6H4-44352-00-00
- 1 x Impeller key — Yamaha reference 90280-03M03-00
- 1 x Insert cup — Yamaha reference 663-44322-00-00
- 1 x Wear plate — Yamaha reference 663-44323-00-00
- 1 x Wear-plate-to-housing gasket — Yamaha reference 663-44315-A0-00
- 1 x Wear-plate-to-base gasket — Yamaha reference 663-44324-A0-00
- 1 x Water-pump gasket — Yamaha reference 663-44316-00-00
- 1 x Water seal — Yamaha reference 93101-23070-00
- 2 x Short water-pump bolts — Yamaha reference 97095-08045-00
- 2 x Long water-pump bolts — Yamaha reference 97095-08055-00
Not included: water-pump housing. Housing-related components from the separate Sierra 18-3408 with-housing kit are not part of this package.
Installation Notes
- Disconnect the battery and follow the outboard manufacturer’s safety procedures before beginning service.
- Place the outboard in neutral and remove the lower unit according to the service manual for your exact model.
- Remove the water-pump cover or housing and note the orientation of the impeller, key, cup, wear plate, gaskets, and seal.
- Remove the old impeller, key, cup, wear plate, gaskets, seal, and pump bolts. Clean all mating surfaces thoroughly.
- Inspect the original water-pump housing for cracks, distortion, corrosion, and scoring. Do not reuse a damaged housing.
- Install the new cup, wear plate, gaskets, water seal, and key in the correct order for your engine.
- Fit the new impeller over the key and driveshaft. Rotate the driveshaft in the manufacturer-specified direction while seating the impeller vanes in the housing.
- Reinstall the housing and tighten the supplied bolts to the torque specification in the applicable Yamaha service manual.
- Reinstall the lower unit, ensuring correct alignment of the driveshaft, shift shaft, and water tube.
- Test the engine in water or with the appropriate flushing equipment and verify a strong cooling-water discharge before normal operation.
Troubleshooting & FAQ
How do I confirm this kit fits my Yamaha engine?
Match the complete model code and year to the compatibility information, then compare the water-pump reference on your existing parts or service manual. Yamaha 40 HP and 50 HP engines can use different water-pump kits depending on model code, year, and lower-unit configuration.
Does this kit include the water-pump housing?
No. This is the without-housing version. The original housing must be serviceable, or a compatible replacement housing must be purchased separately.
What are the signs of a failing water pump?
Common signs include a weak or intermittent telltale stream, overheating warnings, elevated operating temperature, or visible impeller damage such as cracked, brittle, or missing vanes.
What should I inspect before installing the kit?
Inspect the housing, wear surfaces, water tube, keyway, and mating surfaces. A scored or distorted housing can prevent the new impeller from producing proper cooling-water flow.
How often should the impeller be replaced?
Inspect the impeller during scheduled maintenance and replace it when the manufacturer’s service interval is reached or whenever wear, brittleness, cracking, or vane deformation is found. Operating in sandy, silty, or debris-filled water may require more frequent inspection.
Is this an OEM Yamaha part?
No. This listing is for an aftermarket replacement kit. The Yamaha numbers shown are OEM reference numbers used for identification and cross-reference purposes.
Safety & Important Information
- Confirm the complete engine model, year, and existing part references before ordering.
- Do not operate an outboard with a damaged or improperly installed water pump.
- Do not reuse a cracked, warped, or heavily scored housing with a new impeller.
- Use the correct service-manual torque specifications and installation sequence for your exact engine.
- Professional installation is recommended if you are unfamiliar with lower-unit removal or outboard cooling-system service.
About Sea Sierra
- Marine Replacement Parts — Practical aftermarket components for routine outboard service and repair.
- Application-Focused Information — Product references and compatibility details are provided to help you compare the part with your engine.
- Service-Ready Kits — Replacement components are grouped for efficient water-pump maintenance when the original housing remains serviceable.
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