Outboard Thermostat for Mercury/Mariner, Tohatsu/Nissan & Evinrude/Johnson – 50°C (122°F)
Outboard Thermostat for Mercury/Mariner, Tohatsu/Nissan & Evinrude/Johnson – 50°C (122°F) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
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Outboard Thermostat for Mercury/Mariner, Tohatsu/Nissan & Evinr...
Outboard Thermostat for Mercury/Mariner, Tohatsu/Nissan & Evinrude/Johnson – 50°C (122°F) is backordered and will ship as soon as it is back in stock.
Product Details
This aftermarket outboard thermostat is designed for selected Mercury/Mariner, Tohatsu/Nissan, and Evinrude/Johnson marine engines. It helps regulate engine temperature at the specified 50°C (122°F) operating point for the compatible applications listed below.
The thermostat is sold as a single replacement component. It is listed for applications using Mercury references 804540 and 853704, Tohatsu references 3H6-01030-0 and 345-01030-0, Evinrude/Johnson reference 5040958, and Sierra reference 18-3638.
- Replacement thermostat for selected outboard cooling systems.
- Temperature specification: 50°C (122°F).
- One thermostat included.
- Confirm the complete engine model, horsepower, stroke, and serial-number range before ordering.
Part Number Matrix
The following manufacturer and aftermarket references are listed for identification and cross-reference purposes. Always compare the original part and engine application before installation.
| Brand | Reference |
|---|---|
| Mercury | 804540 |
| Mercury | 853704 |
| Tohatsu/Nissan | 3H6-01030-0 |
| Tohatsu/Nissan | 345-01030-0 |
| Tohatsu/Nissan | 3H6010300 / 3H6010300M |
| Evinrude/Johnson | 5040958 |
| Sierra | 18-3638 |
Engine Compatibility
Company fitment information identifies the following application groups. Model, stroke, and serial-number conditions must remain together when checking compatibility; horsepower alone is not sufficient.
| Brand | Application | Horsepower | Stroke / Configuration | Serial Condition | Reference |
|---|---|---|---|---|---|
| Mercury/Mariner | Selected carbureted models | 2.5–3.5 HP | 4-Stroke | 0R131842 and up | 804540 |
| Mercury/Mariner | Selected models | 4–6 HP | 4-Stroke | 0R411838 and up | 804540 |
| Mercury/Mariner International | Two-cylinder, 430 cc applications | 25–30 HP | Confirm the engine model and stroke | Confirm the engine serial number | 804540 / 853704 |
| Brand | Application Range | Horsepower | Reference | Notes |
|---|---|---|---|---|
| Tohatsu/Nissan | Selected marine outboard applications | 3.5–40 HP | 3H6-01030-0 / 345-01030-0 | Verify the exact model and serial number. |
| Brand | Application | Horsepower | Reference | Notes |
|---|---|---|---|---|
| Evinrude/Johnson | Selected outboard applications | 3, 4, and 6 HP | 5040958 | Verify the exact model, year, and serial number. |
Technical Specifications
- Product Type
- Outboard engine thermostat
- Temperature Specification
- 50°C (122°F)
- Installation Location
- Engine cooling system / thermostat housing
- Product Format
- Single replacement component
- Replacement References
- Mercury 804540 and 853704; Tohatsu/Nissan 3H6-01030-0 and 345-01030-0; Evinrude/Johnson 5040958; Sierra 18-3638
What's in the Box
- 1 x Outboard thermostat
A gasket, seal, or additional installation hardware is not specified as included. Reuse only components that are confirmed serviceable, or obtain replacement sealing parts separately when required by the engine service procedure.
Installation Notes
- Allow the engine to cool completely before beginning work.
- Prevent accidental starting and follow the safety instructions in the engine service manual.
- Locate the thermostat housing and confirm that the replacement part matches the original component.
- Remove the housing cover according to the manufacturer's procedure.
- Remove the old thermostat and inspect the housing, sealing surfaces, and related components for damage or deposits.
- Install the replacement thermostat in the same orientation as the original part.
- Install a suitable new gasket or seal if required by the engine service manual. No gasket is specified as included with this product.
- Reinstall the housing cover and tighten fasteners to the manufacturer's specified torque.
- Run the engine using the correct test procedure and check for leaks and proper cooling-system operation.
Troubleshooting & FAQ
What temperature is this thermostat specified for?
The company specification is 50°C (122°F). This is the listed temperature specification for the compatible applications.
How do I confirm that it fits my engine?
Compare your original thermostat reference with the Part Number Matrix, then verify the engine brand, exact model, horsepower, stroke or configuration, year where applicable, and serial-number range. For Mercury/Mariner applications, the listed serial conditions include 0R131842 and up for selected 2.5–3.5 HP 4-stroke models and 0R411838 and up for selected 4–6 HP 4-stroke models.
Does this listing include a gasket or housing?
No. The package contains one thermostat only. A gasket, seal, housing, or other installation hardware is not specified as included.
What are common symptoms of a thermostat problem?
Possible symptoms include overheating, abnormal operating temperature, or an engine that runs cooler than expected. Cooling-system problems can have several causes, so inspect water flow, passages, seals, and related components before replacing the thermostat.
Is this an OEM part?
This is an aftermarket replacement component. The manufacturer references shown above are provided for identification and compatibility cross-reference only; this product is not represented as a genuine OEM component.
Safety & Important Information
- Do not rely on horsepower alone to determine fitment.
- Verify the exact engine model, stroke or configuration, year where applicable, and serial number before ordering.
- Compare the replacement part with the original thermostat and inspect all sealing surfaces before installation.
- Use the engine manufacturer's service procedure and specified torque values.
- Professional installation is recommended when the correct procedure or application is uncertain.
About Sea Sierra
- Marine Replacement Parts — Practical replacement components for selected marine engine applications.
- Application-Focused Listings — Reference numbers and fitment conditions are provided to support accurate part selection.
- Support for Maintenance Work — Confirm your engine details and original part before ordering or installing.
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