Thermostat Kit for Mercury Marine 4-Stroke 8–30 HP Outboards – Replaces 855676002, 8M0119207, 8M0139434 and 855676005
Thermostat Kit for Mercury Marine 4-Stroke 8–30 HP Outboards – Replaces 855676002, 8M0119207, 8M0139434 and 855676005 is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
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Thermostat Kit for Mercury Marine 4-Stroke 8–30 HP Outboards – Repl...
Thermostat Kit for Mercury Marine 4-Stroke 8–30 HP Outboards – Replaces 855676002, 8M0119207, 8M0139434 and 855676005 is backordered and will ship as soon as it is back in stock.
Product Details
This thermostat kit is designed as a replacement service solution for selected Mercury Marine 4-Stroke 8–30 HP outboard engines. A properly functioning thermostat helps regulate engine operating temperature and supports consistent cooling-system performance.
The kit is intended for selected engines that use the applicable thermostat specification and replaces the Mercury part numbers listed below. Always compare the part number, thermostat housing and gasket arrangement with the part removed from your engine before ordering.
- Aftermarket replacement thermostat kit for selected Mercury Marine 4-Stroke outboards.
- Designed to help maintain stable operating temperature.
- Includes the thermostat, cover gasket and thermostat seals shown in the package contents.
- Confirm the exact engine model, serial range and original part number before installation.
Part Number Matrix
The following numbers are replacement or cross-reference references associated with this thermostat application. Cross-reference numbers should be used together with engine model and serial-range verification.
| Brand | Part Number | Reference |
|---|---|---|
| Mercury | 855676002 | Replacement reference |
| Mercury | 8M0119207 | Replacement reference |
| Mercury | 8M0139434 | Replacement reference |
| Mercury | 855676005 | Replacement reference |
| Tohatsu | 3NV010300M | Reference for selected applications; verify before ordering |
| Tohatsu | 3R3-01030-0 | Reference for selected applications; verify before ordering |
| Tohatsu | 3R3010300M | Reference for selected applications; verify before ordering |
| Sierra | 18-43175 | Aftermarket cross-reference |
| Mallory | 9-43175 | Aftermarket cross-reference |
Engine & Model Compatibility
This thermostat kit is intended for selected Mercury Marine 4-Stroke 8–30 HP outboards, including applications using the 209 cc engine platform. The exact fit can vary by model year, engine configuration and serial number. The table below is an application guide rather than a substitute for checking the original part number.
| Brand | Application | Rated HP | Stroke | Applicable Reference | Fitment Note |
|---|---|---|---|---|---|
| Mercury | Selected 4-Stroke outboards | 8 | 4-Stroke | 855676002 / 8M0119207 / 8M0139434 / 855676005 | 209 cc, 2-cylinder applications; verify model and serial range |
| Mercury | Selected 4-Stroke outboards | 9.9 | 4-Stroke | 855676002 / 8M0119207 / 8M0139434 / 855676005 | 209 cc, 2-cylinder applications; verify model and serial range |
| Mercury | Selected 4-Stroke outboards | 15 | 4-Stroke | 855676002 / 8M0119207 / 8M0139434 / 855676005 | Selected 209 cc applications; verify model and serial range |
| Mercury | Selected 4-Stroke outboards | 20 | 4-Stroke | 855676002 / 8M0119207 / 8M0139434 / 855676005 | Selected 209 cc applications; verify model and serial range |
| Mercury | Selected 4-Stroke outboards | 25 | 4-Stroke | 855676002 / 8M0119207 / 8M0139434 / 855676005 | Selected 209 cc applications; verify model and serial range |
| Mercury | Selected 4-Stroke outboards | 30 | 4-Stroke | 855676002 / 8M0119207 / 8M0139434 / 855676005 | Selected 209 cc applications; verify model and serial range |
Selected Tohatsu and Nissan 4-Stroke outboards may use related thermostat references. Confirm the engine model, year, serial number and original thermostat before purchase; horsepower alone does not establish fitment.
Technical Specs
- Product Type
- Aftermarket thermostat repair kit
- Application
- Selected Mercury Marine and related 4-Stroke outboard applications
- Rated Engine Range
- 8–30 HP, subject to model and serial-range verification
- Installation
- Direct replacement when the listed reference and housing configuration match
- Temperature Rating
- 60°C (140°F)
- Package Weight
- 0.08 lb (0.036 kg)
What’s in the Box
- 1 x Thermostat
- 1 x Thermostat cover gasket
- 2 x Thermostat seals
Installation Notes
- Turn the engine off, allow it to cool completely and remove the ignition key. Follow the safety procedures in the engine service manual.
- Disconnect the battery where required by the manufacturer and place the outboard in a stable service position.
- Locate the thermostat housing and note the position and length of each fastener.
- Remove the housing cover carefully. Water may remain in the cooling passages, so protect the surrounding area from spills.
- Remove the old thermostat, gasket and seals, noting the thermostat orientation.
- Clean the housing and cover mating surfaces. Remove old gasket material and corrosion without damaging the sealing surfaces.
- Install the replacement thermostat in the same orientation as the original, then position the new gasket and seals correctly.
- Reinstall the housing cover and tighten the fasteners to the manufacturer's specified torque and sequence.
- Restore any removed connections and run the engine using the correct water-supply method for the outboard. Check for leaks and confirm normal water discharge and temperature behavior.
- If the engine overheats, has weak water discharge or shows a warning, stop the engine immediately and inspect the installation and cooling system.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Possible symptoms include overheating, unusually low operating temperature, unstable temperature behavior, reduced performance or a thermostat that is visibly corroded or stuck. Inspect the thermostat and the rest of the cooling system before replacing parts.
What happens if the thermostat is stuck open or closed?
A thermostat stuck closed can restrict cooling flow and contribute to overheating. A thermostat stuck open can prevent the engine from reaching its intended operating temperature. Either condition should be diagnosed promptly.
What is the thermostat temperature rating?
This listing identifies a 60°C (140°F) thermostat rating. Verify the required temperature specification for your exact engine model and serial range before installation.
How often should an outboard thermostat be replaced?
Replacement intervals depend on the manufacturer's maintenance schedule, operating hours and water conditions. Inspect the thermostat when performing scheduled cooling-system service and replace it if it is corroded, stuck or damaged.
Can I install this thermostat kit myself?
Installation may be straightforward for an experienced boat owner, but correct orientation, gasket placement, fastener torque and post-installation cooling-system checks are important. Use the applicable service manual or consult a qualified marine technician if you are unsure.
Is this a direct replacement for my original thermostat?
It is designed as a direct replacement only when your original part number, engine application and housing configuration match the references listed on this page. Confirm fitment before ordering.
Is this an OEM part?
No. This is an aftermarket replacement kit and is not manufactured by Mercury Marine, Tohatsu, Nissan, Sierra or Mallory.
What is included with the kit?
The package includes one thermostat, one thermostat cover gasket and two thermostat seals. No additional engine parts, tools or installation hardware are included unless specifically listed above.
Shipping speed and cost
Standard shipping is free where offered. Typical transit is 5–15 days, while available expedited options and delivery estimates are shown at checkout.
Warranty and returns
Warranty coverage and return eligibility are subject to the applicable store policies. Contact customer support with your order details if you receive a damaged or incorrect part.
Safety & Important Information
- Verify the engine brand, model, year, serial range, original part number and thermostat housing before ordering.
- Do not rely on horsepower alone to establish compatibility.
- Never work on a hot engine or operate an outboard without the correct water supply.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended when the service procedure or fitment is uncertain.
About Sea Sierra
- Pro-Grade Standards — Marine replacement parts selected for dependable fit and service performance.
- Fast Global Fulfillment — Free standard delivery to most regions where offered.
- Manufacturer-Direct Value — Competitive pricing with practical aftermarket replacement options.
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