Mercury Mariner 140°F Thermostat – 8, 9.9, 15, 25, 30 & 40 HP 4-Stroke Outboards
Mercury Mariner 140°F Thermostat – 8, 9.9, 15, 25, 30 & 40 HP 4-Stroke Outboards is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
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Mercury Mariner 140°F Thermostat – 8, 9.9, 15, 25, 30 & 40 HP 4...
Mercury Mariner 140°F Thermostat – 8, 9.9, 15, 25, 30 & 40 HP 4-Stroke Outboards is backordered and will ship as soon as it is back in stock.
Product Details
This 140°F (60°C) thermostat is a direct-replacement cooling-system component for selected Mercury and Mariner 4-stroke outboard motors rated at 8, 9.9, 15, 25, 30 and 40 HP. The thermostat helps the engine reach and maintain its intended operating temperature when the cooling system is functioning correctly.
Use the original thermostat, engine model, horsepower, stroke, and serial-number range to confirm an exact match before ordering. The same reference numbers may appear across multiple applications, so horsepower alone is not sufficient for final fitment verification.
- 140°F (60°C) operating temperature rating.
- Designed for selected Mercury and Mariner 4-stroke outboards.
- Single thermostat replacement component; package contents are limited to the items supplied with the selected product.
- Aftermarket replacement, not a Mercury OEM-branded component.
Part Number Matrix
The following are manufacturer and aftermarket reference numbers associated with this thermostat application. They are provided for identification and cross-reference purposes.
| Brand or Reference | Part Number |
|---|---|
| Mercury | 825212A1 |
| Mercury | 825212 |
| Mercury | 825212001 |
| Mercury | 855676003 |
| Mercury | 825212T02 |
| Mercury | 855676A1 |
| Sierra | 18-3642 |
Engine & Model Compatibility
Selected compatibility information is listed below. Confirm the complete model designation and serial-number range against the engine identification label or the manufacturer service information before installation.
| Brand | Engine Rating | Stroke | Application Notes | Reference |
|---|---|---|---|---|
| Mercury / Mariner | 8 HP | 4-Stroke | Selected models; verify model and serial number. | 825212A1 |
| Mercury / Mariner | 9.9 HP | 4-Stroke | Selected models; verify model and serial number. | 825212A1 |
| Mercury / Mariner | 15 HP | 4-Stroke | Selected models; verify model and serial number. | 825212A1 |
| Mercury / Mariner | 25 HP | 4-Stroke | Selected models; verify model and serial number. | 825212A1 |
| Mercury / Mariner | 30 HP | 4-Stroke | Selected models; verify model and serial number. | 825212A1 |
| Mercury / Mariner | 40 HP | 4-Stroke | One catalog reference identifies a 40 HP, 3-cylinder application from serial 1C049739 and up; verify the exact engine before ordering. | 825212A1 |
Technical Specs
- Product Type
- Thermostat
- Application
- Selected Mercury and Mariner 4-stroke outboard motors
- Temperature Rating
- 140°F (60°C)
- Rated Engine Range
- 8, 9.9, 15, 25, 30 and 40 HP
- Installation
- Direct replacement when matched to the correct engine model and serial range
- Product Construction
- Thermostat replacement component
What's in the Box
- 1 x Thermostat
Seals, gaskets, housing components and installation materials are not specified as included with this single thermostat listing. Reuse or replace related sealing components only as required by the engine service procedure.
Installation Notes
- Allow the outboard to cool completely before beginning work and follow all applicable marine-service safety procedures.
- Confirm that the replacement thermostat matches the original part and the engine's model, horsepower, stroke and serial-number requirements.
- Remove the thermostat cover according to the engine service manual. Keep track of fasteners and note the original thermostat orientation.
- Remove the old thermostat and inspect the housing, mating surfaces and sealing components for corrosion, damage or debris.
- Clean the mating surfaces without scratching them. Do not use sealant unless the engine manufacturer's service instructions specifically require it.
- Install the thermostat in the correct orientation. Replace damaged or hardened seals or gaskets with the correct service parts.
- Reinstall the cover and tighten the fasteners evenly to the manufacturer's specified torque.
- Run the engine in a safe test environment with adequate cooling-water supply, then check for proper water flow, leaks and normal operating temperature.
Troubleshooting & FAQ
What symptoms can indicate a thermostat problem?
Possible symptoms include overheating, slow warm-up, operation that remains unusually cool, irregular temperature behavior or restricted cooling-water flow. These symptoms can also be caused by impeller, water-passage, sensor or other cooling-system faults, so inspect the complete system.
What temperature is this thermostat rated for?
The stated rating is 140°F (60°C). Always compare this value with the specification for your exact engine model before installation.
Is this a thermostat kit?
No. This listing is for a single thermostat replacement component. A housing gasket, rubber seal or other service part is not identified as included unless separately stated in the order or packaging.
Does it fit every Mercury or Mariner 4-stroke engine in this horsepower range?
No. The listed horsepower range identifies the intended application group, but exact fitment can vary by model, production version and serial-number range. Verify all engine-identification details before ordering.
Do I need to replace the gasket or seal?
Inspect the existing sealing components during service and replace any damaged, compressed or hardened parts with the correct components for the engine. This listing does not specify a gasket or seal as included.
Is this an OEM part?
This is an aftermarket replacement. The manufacturer reference numbers shown are provided for identification and compatibility cross-reference; the product is not presented as a Mercury OEM-branded component.
Safety & Important Information
- Verify the engine model, horsepower, stroke and serial-number range before purchase.
- Do not operate an outboard without an adequate cooling-water supply.
- Inspect the thermostat housing and cooling passages for corrosion or blockage during service.
- Professional installation is recommended when the correct service procedure or torque specification is unavailable.
About Sea Sierra
- Marine Replacement Parts — Practical replacement components for selected outboard service applications.
- Fitment-Focused Listings — Reference numbers and application details are provided to help match the correct part.
- Support for Routine Maintenance — Use the correct service information for your exact engine before installation.
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