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)

This water pump repair kit is designed for select Yamaha 2-stroke outboard engines, including 40HP (1984–1994), 50HP (1984–1987), Pro Series 50HP (1987–1990), and P50 models (1995–1996). It provides the necessary components to overhaul your engine's cooling system for reliable performance.

This kit serves as a direct replacement for Yamaha part numbers 6H4-W0078-A0-00, 6H4-W0078-00-00, and 6H4-W0078-01-00. It is also an aftermarket equivalent to Sierra 18-3429. For a kit that includes the housing, reference Sierra part 18-3408.

• Installation Method: Installs as a direct OEM-style replacement.
• Engine Compatibility: Designed for marine 2-stroke outboard cooling systems.
• Complete Kit: Includes a new impeller, gaskets, wear plate, insert cup, water seal, and other hardware required for a standard rebuild.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Yamaha owners, here’s a guide to help you identify your engine’s serial/model: Yamaha Serial Number Guide. You can also review exploded diagrams to verify part numbers: Yamaha Parts Diagrams & Catalog.

Technical Specifications

Package Contents

• 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)
• 2 x Bolt (97095-08045-00)
• 2 x Bolt (97095-08045-00)

Installation Guide

1. Safety First: Disconnect the battery and ensure the engine is in a secure, stable position.
2. Drain Gear Lube: Place a drain pan under the lower unit and remove the upper and lower gear lube drain screws to drain the oil.
3. Remove Lower Unit: Remove the mounting bolts securing the lower unit (gearcase) to the midsection. Carefully lower it to access the water pump.
4. Disassemble Pump: Unscrew the bolts on the water pump housing and lift it off. You may need to gently pry it.
5. Remove Old Components: Take out the old impeller, wear plate, gaskets, and seals. Note their orientation for correct reassembly.
6. Clean Surfaces: Thoroughly clean all mating surfaces, removing any old gasket material or corrosion.
7. Install New Parts: Place the new gasket and outer plate on the gearcase. Install the new insert cup into the housing.
8. Install Impeller: Slide the new Woodruff key onto the driveshaft and install the new impeller, turning the driveshaft clockwise to bend the vanes correctly.
9. Reassemble Pump: Apply a light coat of marine grease to the inside of the insert cup. Slide the housing down over the impeller and secure it with the new bolts and gaskets.
10. Reattach Lower Unit: Guide the lower unit back into place, ensuring the driveshaft, shift shaft, and water tube align correctly. Fasten the mounting bolts.
11. Refill Gear Lube: Fill the lower unit with fresh gear lube from the bottom drain hole until it seeps out of the top hole, then reinstall both screws.
12. Test Operation: Securely attach a water source ("ear muffs") and run the engine to check for proper water flow and no leaks.

Troubleshooting & FAQ

How do I confirm this kit fits my engine?
First, check the "Compatible Engine Models" table on this page. Your engine's model, horsepower, and year range should be listed. Second, compare the part numbers in the "Part Number Cross-Reference" table to your existing part or service manual. Finally, visually compare our product photos with your old parts before ordering.

What are the signs that my water pump needs to be replaced?
Common signs of a failing water pump include the engine overheating, a weak or inconsistent stream of water from the "telltale" outlet, and visible cracking or deterioration of the impeller vanes upon inspection.

How often should I replace my water pump impeller?
For preventative maintenance, it is recommended to replace your impeller every 1-2 years or every 100-200 hours of operation, depending on usage conditions (e.g., saltwater vs. freshwater). Always consult your engine's service manual for specific intervals.

Does this kit include the water pump housing?
No, this is a basic repair kit that does not include the metal water pump housing. This kit matches Sierra 18-3429. If your housing is scored, warped, or cracked, you should purchase and install a new one separately. The kit that includes a housing is typically equivalent to Sierra 18-3408.

OEM vs. Aftermarket?
This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

How fast do you ship and how much does it cost?
We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

What is your warranty and return policy?
This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.