Thermostat – Fits Yamaha 9.9-70 HP 4-Stroke Outboards (Replaces 6G8-12411-00, 69M-12411-00)
Thermostat – Fits Yamaha 9.9-70 HP 4-Stroke Outboards (Replaces 6G8-12411-00, 69M-12411-00) is backordered and will ship as soon as it is back in stock.
Worry-free returns
Worry-free returns
Shop with confidence. If you’re not completely satisfied, eligible items may be returned within the return window, provided they are unused and in original packaging.
If the return is due to our error or a defective item, we’ll make it right.
Please contact us before returning any item.
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.
International delivery times may vary due to customs processing.
Tracking information will be provided once your order ships.
Thermostat – Fits Yamaha 9.9-70 HP 4-Stroke Outboards (Replaces 6G8...
Thermostat – Fits Yamaha 9.9-70 HP 4-Stroke Outboards (Replaces 6G8-12411-00, 69M-12411-00) is backordered and will ship as soon as it is back in stock.
Product Details
This replacement Thermostat – Fits Yamaha 9.9-70 HP 4-Stroke Outboards and is designed to regulate engine temperature for optimal performance and longevity. It is compatible with a wide range of Yamaha 4-stroke models from 1987 to the present, including the F9.9, F15, F20, F25, F40, F50, F60, and F70 series.
It serves as a direct interchange for Yamaha part numbers 6G8-12411-00, 6G8-12411-03-00, 66M-12411-01-00, 69M-12411-00, 69M-12411-01, 6AH-12411-00, and Sierra 18-3624.
- Engineered as a direct, drop-in replacement for the original OEM thermostat.
- Opens at the factory-specified temperature of 50°C (122°F) to ensure correct engine operation.
- The kit includes a new thermostat, gasket, and o-ring for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket brands.
| Brand | Part Number | Superseded By | Notes |
|---|---|---|---|
| Yamaha | 6G8-12411-00 | Primary reference number. | |
| Yamaha | 6G8-12411-03-00 | 66M-12411-01-00 | |
| Yamaha | 66M-12411-01-00 | Used on F70 models. | |
| Yamaha | 69M-12411-00 | ||
| Yamaha | 69M-12411-01 | ||
| Yamaha | 6AH-12411-00 | Used on F20B models. | |
| Sierra | 18-3624 | Aftermarket cross-reference. |
Engine & Model Compatibility
Confirm your engine model and year in the table below. This thermostat fits a broad selection of Yamaha 4-stroke outboards.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | F2.5MSH-0405, F2.5MSH-0406, F2.5MSH-0407, F2.5MSH0608, F2.5MSHD-2005 | 2.5 | 4-Stroke | 6G8-12411-00 | ||
| Yamaha | F9.9H (6CA) | 9.9 | 4-Stroke | 2009+ | 6G8-12411-00 | |
| Yamaha | F15C (6AG) | 15 | 4-Stroke | 2007+ | 6G8-12411-00 | |
| Yamaha | F20B (6AH) | 20 | 4-Stroke | 2007+ | 6G8-12411-00, 6AH-12411-00 | |
| Yamaha | F70 | 70 | 4-Stroke | 66M-12411-01-00 | Supersedes 6G8-12411-03-00 | |
| Yamaha | Various models | 9.9-70 | 4-Stroke | 1987+ | 69M-12411-00, 69M-12411-01 | General compatibility range |
Technical Specs
- Product Type
- Thermostat
- Installation
- Direct replacement
- Thermostat Opening Temperature
- 50°C (122°F)
- Material
- Corrosion-resistant alloy with precision-engineered sealing components
- Operating Temperature Range
- 50°C (122°F) opening temperature
- Other Technical Attributes
- Spring assembly faces downwards/towards engine block in most installations
What’s in the Box
- 1 × 50°C / 122°F Outboard Thermostat
- 1 × Gasket
- 1 × O-ring
Installation Notes
- Ensure the engine is off and cool. Disconnect the battery for safety.
- Place a drain pan under the engine and drain the coolant system if necessary.
- Locate the thermostat housing on the engine block or cylinder head.
- Remove the bolts securing the housing cover. Note their positions if they are different lengths.
- Carefully remove the old thermostat and its gasket or o-ring. Clean any residue from the housing and cover surfaces.
- Insert the new thermostat, ensuring the spring assembly faces down towards the engine block.
- Place the new gasket and/or o-ring in position and reinstall the housing cover.
- Tighten the bolts to the manufacturer's specified torque in a crisscross pattern. Do not overtighten.
- Refill the cooling system with the recommended coolant.
- Reconnect the battery, start the engine, and check for leaks and proper temperature gauge operation.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs of a bad thermostat include engine overheating at idle or low speeds, the temperature gauge reading unusually high or low, or the engine failing to reach its normal operating temperature. A visual inspection may show corrosion or a stuck spring.
What is the correct installation orientation?
In most Yamaha applications, the spring-loaded end of the thermostat should face downwards, towards the engine block. Always verify with your specific service manual if you are unsure.
Is 50°C (122°F) the correct temperature for my engine?
Yes, this 50°C (122°F) opening temperature is the standard specification for the listed Yamaha 4-stroke models. It ensures the engine warms up quickly and maintains a stable operating temperature for efficiency and longevity.
Why does the F70 model sometimes use a different part number?
Yamaha has updated part numbers over time. Part number 6G8-12411-03-00, which was used on some models, has been officially superseded by 66M-12411-01-00. This thermostat kit is compatible with both references.
How often should I replace my outboard's thermostat?
As a preventative maintenance item, it is recommended to replace the thermostat every 2-3 years or 200-300 hours, especially if operating in saltwater. Consult your engine's service manual for specific intervals.
Does this kit include everything required for the job?
Yes, this kit includes the thermostat, a new gasket, and an o-ring. This provides all the necessary components for a standard replacement.
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.
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 vs 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.
Payment & Security
Payment methods
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
