Water Pump Wear Plate – Fits Yamaha 25HP 30HP 40HP 50HP 55HP (Replaces 663-44323-00, 18-3436, 81420M)

Product Details

This Water Pump Wear Plate is engineered as a direct replacement for Yamaha outboard lower units ranging from 25HP to 55HP, covering both 2-stroke engines manufactured between 1984 and 1996. Compatible models include Yamaha 25J, 25JEO, 25ESH, 25Q, 25QEO, 30DE, 30DEO, 30D, 30DMH, 30DEL, 30ELH, 40N, 40H, 40HE, 40QE, 40JWH, 40QM, 40QWC, 40JMH, 50HP, 50ETN, 50D, 50DEO, 50DEMTO, 50 Pro Series, P50, 55ED, 55ET, 55B, 55BM, and 55BT.

This aftermarket wear plate directly replaces OEM part number 663-44323-00 and cross-references with Sierra 18-3436 and Mercury Marine 81420M.

• Essential water pump component designed to protect the impeller and housing
• Precision-engineered for proper clearance and flow in Yamaha lower units
• Drop-in installation during routine water pump service or rebuild

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Engine & Model Compatibility

Confirm your engine model/year in the table below.

Technical Specs

OEM Part Number

663-44323-00

Replaces

Sierra 18-3436, Mercury Marine 81420M

Application

Yamaha Outboard Lower Unit Water Pump

Engine Type

2-Stroke

Horsepower Range

25HP – 55HP

What's in the Box

• 1× Impeller Wear Plate: 663-44323-00

Installation Notes

1. Disconnect the battery and remove the lower unit from the outboard following the manufacturer's service manual.
2. Remove the existing water pump housing and carefully extract the old impeller and wear plate.
3. Clean all mating surfaces thoroughly, removing old gasket material and debris.
4. Position the new wear plate in the pump housing with the correct side facing the impeller.
5. Install the impeller, ensuring the vanes are oriented correctly per the rotation direction indicated in the housing.
6. Apply marine-grade grease to the impeller vanes to aid initial priming and prevent dry starts.
7. Reassemble the water pump housing with a new gasket set and torque fasteners to manufacturer specifications.
8. Reinstall the lower unit and reconnect all fittings and shift linkages.
9. Test the cooling system by running the engine in a test tank or on-water; verify steady water discharge from the tell-tale.

Troubleshooting & FAQ

How do I know if my wear plate needs replacement?

Inspect the wear plate during routine water pump service. Replace if you notice deep grooves, cracks, warping, or excessive wear that could allow the impeller to contact metal surfaces. Most technicians replace the wear plate whenever the impeller is changed.

Will this fit all Yamaha 40HP–50HP engines?

This wear plate fits specific 2-stroke Yamaha models from 1984–1996 as listed in the compatibility table. Always cross-reference your engine's serial number and original part number to confirm fitment before ordering.

Can I reuse the old wear plate when installing a new impeller?

It is not recommended. Worn or grooved wear plates reduce impeller efficiency, increase friction, and can cause premature impeller failure. Always replace the wear plate as part of a complete water pump rebuild.

What happens if the wear plate is installed backwards?

Installing the wear plate in reverse can disrupt water flow, cause impeller damage, and lead to overheating. Ensure the correct orientation per the pump housing markings or service manual.

How often should I replace the wear plate?

Replace the wear plate every time you service the water pump or impeller, typically every 2–3 years or 200–300 hours of operation, whichever comes first. Saltwater use may require more frequent intervals.

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.

Does this come with gaskets or other pump components?

No. This listing includes only the wear plate. Gaskets, impeller, housing, and other water pump components are sold separately. We recommend purchasing a complete water pump rebuild kit for comprehensive service.

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 against 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.