Water Pump Impeller Service Kit – Fits Yamaha 2-Stroke 40/50 HP (Replaces 6H4-W0078-00, 18-3408, 18-3429)
Water Pump Impeller Service Kit – Fits Yamaha 2-Stroke 40/50 HP (Replaces 6H4-W0078-00, 18-3408, 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
Orders are typically processed within 1–2 business days.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
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Water Pump Impeller Service Kit – Fits Yamaha 2-Stroke 40/50 HP (Re...
Water Pump Impeller Service Kit – Fits Yamaha 2-Stroke 40/50 HP (Replaces 6H4-W0078-00, 18-3408, 18-3429) is backordered and will ship as soon as it is back in stock.
Product Details
This Water Pump Impeller Service Kit is designed for Yamaha 2-stroke outboard motors, specifically fitting 40 HP models from 1995–2005 and 50 HP models (including the Pro Series P50) from 1998–2009. It provides all necessary components for a complete water pump overhaul to restore cooling performance.
This kit serves as a direct replacement for Yamaha OEM part numbers 6H4-W0078-00, 6H4-W0078-00-00, 6H4-W0078-01-00, and 6H4-W0078-A0-00, as well as Sierra aftermarket parts 18-3408 and 18-3429.
- Installs in the lower unit of the outboard motor.
- Features a durable neoprene impeller for reliable performance.
- Engineered as a direct OEM-compatible replacement part.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Yamaha | 6H4-W0078-00 |
| Yamaha | 6H4-W0078-00-00 |
| Yamaha | 6H4-W0078-01-00 |
| Yamaha | 6H4-W0078-A0-00 |
| Sierra | 18-3408 |
| Sierra | 18-3429 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For Yamaha owners, please verify your engine's model and production year against this list to ensure proper fitment of the water pump kit.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | 40 HP | 40 | 2-Stroke | 1995–2005 | 6H4-W0078-00 | |
| Yamaha | 50 HP | 50 | 2-Stroke | 1998–2009 | 6H4-W0078-00 | |
| Yamaha | P50 (Pro Series) | 50 | 2-Stroke | 1998–2009 | 6H4-W0078-00 |
Technical Specs
- Product Type
- Water Pump Impeller Service Kit
- Installation
- Lower Unit
- Material
- Neoprene (Impeller)
- Weight
- 0.25 kg (0.55 lb)
- Other Technical Attributes
- Original Equipment Manufacturer (OEM) or OEM-compatible replacement
What’s in the Box
- 1 x Water Pump Impeller (6H4-44352-00)
- 1 x Wear Plate
- 1 x Insert Cup
- 1 x Water Pump Gasket
- 1 x Key
- 1 x Water Seal
- 1 x "O"-Ring
Installation Notes
- Disconnect the spark plug leads and shift the outboard into the neutral position.
- Locate and remove the bolts securing the lower unit (gearcase) to the upper casing.
- Carefully separate and lower the gearcase from the driveshaft housing. Be mindful of the shift shaft.
- Unbolt the water pump housing cover to access the impeller.
- Remove the old impeller, woodruff key, wear plate, and any gaskets or seals.
- Clean the housing and mating surfaces meticulously, removing any old gasket material or corrosion.
- Install the new wear plate and gasket. Place the new woodruff key on the driveshaft.
- Align the keyway and slide the new impeller onto the driveshaft. Turn the shaft clockwise to help seat the vanes.
- Install the new insert cup and gasket onto the housing, then secure the housing with its bolts.
- Re-attach the lower unit to the engine, ensuring the driveshaft, water tube, and shift shaft align correctly. Tighten bolts to the manufacturer's specified torque.
Troubleshooting & FAQ
How do I know this kit fits my Yamaha outboard?
This kit is specifically for Yamaha 2-Stroke 40 HP (1995–2005) and 50 HP (1998–2009) models. The most reliable way to confirm fit is to check your engine's model and year against our compatibility table. You can also match your existing part number to the cross-references listed in the Part Number Matrix.
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, or visible damage (cracked, brittle, or missing vanes) on the old impeller during inspection.
How often should this water pump kit be replaced?
As a preventative maintenance item, it is generally recommended to replace the water pump impeller and service the pump every one to two years, or every 100-200 hours of operation, depending on usage conditions (e.g., saltwater vs. freshwater).
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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