Thermostat 143°F (62°C) – Fits Yamaha 2-Stroke 9.9-55 HP (Replaces 6F5-12411-01, 6F5-12411-02, 6F5-12411-03)
Thermostat 143°F (62°C) – Fits Yamaha 2-Stroke 9.9-55 HP (Replaces 6F5-12411-01, 6F5-12411-02, 6F5-12411-03) is backordered and will ship as soon as it is back in stock.
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Thermostat 143°F (62°C) – Fits Yamaha 2-Stroke 9.9-55 HP (Replaces ...
Thermostat 143°F (62°C) – Fits Yamaha 2-Stroke 9.9-55 HP (Replaces 6F5-12411-01, 6F5-12411-02, 6F5-12411-03) is backordered and will ship as soon as it is back in stock.
Product Details
This Thermostat 143°F (62°C) – Fits Yamaha 2-Stroke 9.9-55 HP is an essential cooling system component designed for a wide range of Yamaha outboards from 1986–1997. It ensures your engine maintains the correct operating temperature for optimal performance and longevity. Compatible models include 9.9/15 HP (1986–1995), C25/C30 (1990–1997), C40/C55 (1990–1997), and various CV models (1989–1990).
It serves as a direct replacement for Yamaha part numbers 6F5-12411-01-00, 6F5-12411-02-00, and 6F5-12411-03-00, and interchanges with Sierra 18-3625 and Quicksilver 814554T01.
- Engineered for direct OEM-replacement fit and function.
- Constructed from high-grade, corrosion-resistant metal for marine environments.
- Package includes the thermostat and a new O-ring gasket for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket suppliers.
| Brand | Part Number | Notes |
|---|---|---|
| Yamaha | 6F5-12411-01-00 | Superseded by 6F5-12411-02-00 |
| Yamaha | 6F5-12411-02-00 | Superseded by 6F5-12411-03-00 |
| Yamaha | 6F5-12411-03-00 | Current OEM Part |
| Sierra Marine | 18-3625 | Aftermarket Interchange |
| Quicksilver | 814554T01 | Crossover Application Interchange |
Engine & Model Compatibility
Confirm your engine model and year in the tables below.
For Yamaha Outboards: This thermostat fits a wide range of 2-stroke models from 9.9 HP to 55 HP manufactured between 1986 and 1997.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. |
|---|---|---|---|---|---|
| Yamaha | 9.9, 9.9EL, 9.9ES | 9.9 | 2-Stroke | 1986–1995 | 6F5-12411-01/02/03 |
| Yamaha | 9.9L, 9.9S | 9.9 | 2-Stroke | 1986–1990 | 6F5-12411-01/02/03 |
| Yamaha | 9.9M | 9.9 | 2-Stroke | 1991–1995 | 6F5-12411-01/02/03 |
| Yamaha | 15, 15EL, 15ES | 15 | 2-Stroke | 1986–1995 | 6F5-12411-01/02/03 |
| Yamaha | 15L, 15S | 15 | 2-Stroke | 1986–1990 | 6F5-12411-01/02/03 |
| Yamaha | C25EL | 25 | 2-Stroke | 1991–1997 | 6F5-12411-01/02/03 |
| Yamaha | C25ML, C25MS | 25 | 2-Stroke | 1993–1997 | 6F5-12411-01/02/03 |
| Yamaha | CV25EL, CV25EM | 25 | 2-Stroke | 1990 | 6F5-12411-01/02/03 |
| Yamaha | C30EL | 30 | 2-Stroke | 1991–1992 | 6F5-12411-01/02/03 |
| Yamaha | CV30EL | 30 | 2-Stroke | 1989–1990 | 6F5-12411-01/02/03 |
| Yamaha | C40EL | 40 | 2-Stroke | 1991–1997 | 6F5-12411-01/02/03 |
| Yamaha | C40MS, C40PL | 40 | 2-Stroke | 1992–1997 | 6F5-12411-01/02/03 |
| Yamaha | C40ML | 40 | 2-Stroke | 1994–1995 | 6F5-12411-01/02/03 |
| Yamaha | CV40EL | 40 | 2-Stroke | 1990 | 6F5-12411-01/02/03 |
| Yamaha | C55EL | 55 | 2-Stroke | 1991–1995 | 6F5-12411-01/02/03 |
| Yamaha | CV55EL | 55 | 2-Stroke | 1989 | 6F5-12411-01/02/03 |
For Mercury/Mariner Outboards (Crossover Application): This thermostat also fits certain legacy Mercury/Mariner models as a crossover part, equivalent to Quicksilver 814554T01.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury/Mariner | 9.9 | 9.9 | 2-Stroke | 1979–1986 | 814554T01 | Crossover application |
| Mercury/Mariner | 15K, 20C, 25C | 15-25 | 2-Stroke | 1988–1998 | 814554T01 | Crossover application |
| Mercury/Mariner | 25K, 40C, K40, 40 | 25-40 | 2-Stroke | 1986–1998 | 814554T01 | Crossover application |
| Mercury/Mariner | 55 | 55 | 2-Stroke | 1984–1985 | 814554T01 | Crossover application |
Technical Specs
- Product Type
- Thermostat
- Temperature Rating
- 62°C (143°F)
- Opening Temperature
- 62°C (143°F)
- Installation
- Direct OEM replacement
- Housing Material
- High-grade corrosion-resistant metal
- Fuel Type
- Gasoline (Petrol)
What’s in the Box
- 1 x Thermostat (Replaces 6F5-12411-03-00)
- 1 x O-ring Gasket
Installation Notes
- Ensure the engine is completely cool before beginning work. Disconnect the battery for safety.
- Locate the thermostat housing on the engine block or cylinder head.
- Place a drain pan under the housing and slowly loosen the bolts to allow coolant to drain.
- Remove the housing cover and carefully note the orientation of the old thermostat before removing it.
- Thoroughly clean the mating surfaces of the housing and cover, removing any old gasket material.
- Install the new O-ring gasket into the groove on the housing or cover.
- Seat the new thermostat in the housing, ensuring it is oriented correctly (spring-side usually towards the engine).
- Reinstall the housing cover and tighten the bolts evenly in a crisscross pattern to the manufacturer's specified torque.
- Reconnect any hoses, refill the cooling system with the appropriate coolant, and inspect for leaks.
- Start the engine and allow it to warm up, checking for proper operating temperature and watching for any leaks.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs of a faulty thermostat include the engine overheating at low speeds, the temperature gauge fluctuating erratically, or the engine not reaching its normal operating temperature (running too cool). A stuck-open thermostat can lead to poor fuel efficiency and carbon buildup, while a stuck-closed thermostat will cause rapid overheating.
Is 62°C (143°F) the correct temperature for my engine?
Yes, this 62°C (143°F) thermostat is built to the OEM specification for the listed Yamaha and Mercury/Mariner models. While some third-party sources may cite slightly different temperatures, 62°C is the confirmed rating for maintaining optimal engine performance and efficiency in these applications.
Why is replacing the O-ring gasket important?
The O-ring gasket creates a watertight seal between the thermostat housing and its cover. Reusing an old, compressed, or brittle gasket will likely result in a coolant leak, which can lead to overheating and engine damage. Always use the new gasket supplied with the thermostat.
How often should a marine thermostat be replaced?
There is no fixed interval, but thermostats are a common wear item. It is good preventative maintenance to replace it every 2-3 years, especially in saltwater environments, or immediately if you notice any signs of cooling system trouble.
What’s the difference between an OEM and an aftermarket part?
This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials. Our parts are manufactured in an ISO 9002 facility and are batch-inspected for quality and consistency.
What are your shipping speeds and costs?
We ship worldwide quickly—typical transit time is 5–15 days. Standard shipping is free on most orders, with expedited options available at checkout where applicable.
What is your warranty and return policy?
This product is covered by a 1-year warranty against manufacturing defects under normal use. If you encounter any issues, please contact our support team with your order details. Returns are accepted within our standard window for unused parts in their original condition. See our full policy page or reach out for instructions.
Safety & Important Info
- Verify images and specifications against your original part before purchase. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement part, not an original equipment manufacturer (OEM) component.
- Professional installation is highly recommended for safety, performance, and to prevent damage.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and rigorously tested for real-world reliability.
- Fast Global Fulfillment — We offer free standard delivery to most regions with an average transit time of 5–15 days.
- Manufacturer-Direct Value — By working directly with manufacturers, we provide fair pricing without compromising on materials or quality control.
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