Aftermarket Water Pump Impeller for Mercury/Mariner and Tohatsu/Nissan Outboards – 2-Stroke 9.9–18 HP, 4-Stroke 9.9–20 HP
Aftermarket Water Pump Impeller for Mercury/Mariner and Tohatsu/Nissan Outboards – 2-Stroke 9.9–18 HP, 4-Stroke 9.9–20 HP 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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Aftermarket Water Pump Impeller for Mercury/Mariner and Tohatsu/Nis...
Aftermarket Water Pump Impeller for Mercury/Mariner and Tohatsu/Nissan Outboards – 2-Stroke 9.9–18 HP, 4-Stroke 9.9–20 HP is backordered and will ship as soon as it is back in stock.
Product Details
This aftermarket water pump impeller is designed for selected Mercury/Mariner and Tohatsu/Nissan marine outboards. The company application data covers Tohatsu/Nissan 2-stroke 9.9, 15, and 18 HP engines; Tohatsu 4-stroke MFS9.9, MFS15, and MFS18 engines; Tohatsu 4-stroke 20 HP engines; and Mercury/Mariner 4-stroke 9.9 HP 209cc, 15 HP, and 20 HP applications.
Use the reference numbers and model information below as a fitment aid. Outboard water pump components can vary by model family, production year, and serial-number range, so confirm the existing impeller or manufacturer part number before ordering.
- Replacement water pump impeller for selected 2-stroke and 4-stroke outboards.
- Aftermarket replacement listed under Sierra reference 18-8921.
- Sold as one impeller only; an impeller key is not included unless specifically stated by the seller.
- Not an OEM component. Brand names and part numbers are used only to identify intended applications and reference compatibility.
Part Number Matrix
The following manufacturer and aftermarket references are listed for cross-referencing. A reference number alone does not replace confirmation of the complete engine model and serial-number conditions.
| Brand or Source | Reference Number | Use |
|---|---|---|
| Tohatsu | 334-65021-0 | Manufacturer reference |
| Tohatsu | 334650210M | Manufacturer reference format |
| Mercury Marine | 47-803748 | Manufacturer reference |
| Mercury Marine | 8M0135803 | Manufacturer reference |
| Mercury Marine | 8M0214944 | Additional listed reference |
| Mercury Marine | 803748 | Additional listed reference |
| Mercury Marine | 803748K01 | Additional listed reference |
| Sierra | 18-8921 | Aftermarket reference |
| Evinrude/Johnson/OMC/BRP | 5040046 | Additional listed reference |
Engine & Model Compatibility
Compatibility is organized by engine family and stroke type. Confirm the exact model and serial number before installation.
| Brand | Model or Application | Rated HP | Stroke | Reference | Notes |
|---|---|---|---|---|---|
| Tohatsu/Nissan | M9.9, NS9.9 and related applications | 9.9 | 2-Stroke | 334-65021-0 334650210M |
Confirm the exact model designation. |
| Tohatsu/Nissan | M15, NS15 and related applications | 15 | 2-Stroke | 334-65021-0 334650210M |
Confirm the exact model designation. |
| Tohatsu/Nissan | M18, NS18 and related applications | 18 | 2-Stroke | 334-65021-0 334650210M |
Confirm the exact model designation. |
| Tohatsu | MFS9.9 | 9.9 | 4-Stroke | 334-65021-0 334650210M |
Application listed in the company data. |
| Tohatsu | MFS15 | 15 | 4-Stroke | 334-65021-0 334650210M |
Application listed in the company data. |
| Tohatsu | MFS18 | 18 | 4-Stroke | 334-65021-0 334650210M |
Application listed in the company data. |
| Tohatsu | 20 HP applications | 20 | 4-Stroke | 334-65021-0 334650210M |
Confirm the exact model and serial number. |
| Brand | Model or Application | Rated HP | Stroke | Reference | Notes |
|---|---|---|---|---|---|
| Mercury/Mariner | 9.9 HP 209cc applications | 9.9 | 4-Stroke | 47-803748 8M0135803 |
Confirm the 209cc engine configuration. |
| Mercury/Mariner | 15 HP applications | 15 | 4-Stroke | 47-803748 8M0135803 |
Confirm the exact model and serial number. |
| Mercury/Mariner | 20 HP applications | 20 | 4-Stroke | 47-803748 8M0135803 |
Confirm the exact model and serial number. |
Technical Information
- Product Type
- Water pump impeller
- Product Configuration
- Single impeller
- Aftermarket Reference
- Sierra 18-8921
- Primary Manufacturer Reference
- Tohatsu 334-65021-0 / 334650210M
- Additional Manufacturer References
- Mercury 47-803748, 8M0135803, 8M0214944, 803748, 803748K01; Evinrude/Johnson/OMC/BRP 5040046
- Application Range
- Selected 2-stroke 9.9–18 HP and 4-stroke 9.9–20 HP outboards, subject to exact model and serial-number confirmation
What’s in the Box
- 1 x aftermarket water pump impeller
Important: The impeller key is not included with this single-impeller product. Reuse of an old key is not recommended if it is worn, damaged, or does not fit securely in the shaft keyway.
Installation Notes
- Disconnect the battery and follow the outboard manufacturer’s service precautions.
- Place the engine in neutral and remove the lower unit according to the service manual for the specific model.
- Remove the water pump housing and take note of the impeller, key, housing, and base-plate orientation.
- Remove the old impeller and inspect the housing, wear plate, key, and driveshaft keyway for damage or excessive wear.
- Install the correct key for the engine, then align the replacement impeller with the keyway.
- Reinstall the water pump housing using the manufacturer’s specified blade orientation and assembly procedure.
- Reassemble the lower unit, restore the correct lubricant level, and test the cooling-water flow with an appropriate water supply.
- Stop the engine immediately if the tell-tale stream is weak or absent, or if an overheating warning occurs.
Troubleshooting & FAQ
How do I confirm this impeller fits my engine?
Compare your original impeller and manufacturer part number with the reference matrix, then confirm the complete engine brand, model, stroke type, horsepower, and serial-number range. Do not rely on horsepower alone.
Is this a complete water pump kit?
No. This listing is for one water pump impeller only. It does not include an impeller key, water pump housing, wear plate, gasket, or other pump components.
What are common signs of a worn impeller?
A weak or missing tell-tale stream, overheating, or reduced cooling performance can indicate a worn, damaged, incorrectly installed, or obstructed water pump impeller. Inspect the complete cooling system before replacing parts.
Should I replace the impeller key?
Inspect the key and keyway during service. Replace the key if it is worn, damaged, loose, or otherwise unsuitable. The correct key is not included with this single-impeller product.
Is this an OEM part?
No. This is an aftermarket replacement impeller. The listed manufacturer and aftermarket numbers are reference numbers for identifying intended applications and compatibility.
Can the same reference number fit both 2-stroke and 4-stroke engines?
Some catalog references are listed across different engine families, but the complete model and serial-number conditions still determine fitment. Verify the application before installation.
Safety & Important Information
- Verify the original part, reference number, dimensions, and engine application before ordering.
- Do not operate an outboard with insufficient cooling-water flow.
- Follow the manufacturer’s service manual for lower-unit removal, torque values, lubrication, and testing.
- Professional installation is recommended when the correct model, key, or installation procedure is uncertain.
- This is an aftermarket replacement and is not manufactured or endorsed by Mercury, Mariner, Tohatsu, Nissan, Evinrude, Johnson, OMC, BRP, or Sierra.
About Sea Sierra
- Marine Replacement Parts — Practical aftermarket components for routine service and repair.
- Application-Focused Listings — Reference numbers and model information are provided to help customers verify fitment.
- Customer Support — Contact support with your engine model, horsepower, stroke type, serial number, and existing part number when compatibility is uncertain.
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