Thermostat – Fits Yamaha 4-Stroke 80-100HP (Replaces Yamaha 6D9-12411-00, Sierra 18-3639)
Thermostat – Fits Yamaha 4-Stroke 80-100HP (Replaces Yamaha 6D9-12411-00, Sierra 18-3639) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
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Thermostat – Fits Yamaha 4-Stroke 80-100HP (Replaces Yamaha 6D9-124...
Thermostat – Fits Yamaha 4-Stroke 80-100HP (Replaces Yamaha 6D9-12411-00, Sierra 18-3639) is backordered and will ship as soon as it is back in stock.
Product Details
This is a direct replacement Thermostat – Fits Yamaha 4-Stroke 80-100HP (Replaces 6D9-12411-00), designed for reliable temperature regulation in Yamaha F100D, F100A, F100B, and F80A outboard models from 2005–2017. Maintaining the correct engine temperature is crucial for performance and fuel efficiency.
It serves as a direct cross-reference for Yamaha part number 6D9-12411-00 and Sierra part 18-3639.
- Designed for direct-fit installation.
- Maintains a consistent operating temperature of 55°C (131°F).
- Includes one thermostat ready for installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number | Notes |
|---|---|---|
| Yamaha | 6D9-12411-00 | OEM Reference |
| Sierra | 18-3639 | Aftermarket replacement (compatibility unverified by Sea Sierra) |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. |
|---|---|---|---|---|---|
| Yamaha | F100DET | 100 | 4-Stroke | 2005–2014 | 6D9-12411-00 |
| Yamaha | F100L | 100 | 4-Stroke | 2015–2017 | 6D9-12411-00 |
| Yamaha | F100X | 100 | 4-Stroke | 2015–2017 | 6D9-12411-00 |
| Yamaha | F100A | 100 | 4-Stroke | 6D9-12411-00 | |
| Yamaha | F100B | 100 | 4-Stroke | 6D9-12411-00 | |
| Yamaha | F80A | 80 | 4-Stroke | 6D9-12411-00 |
Technical Specs
- Product Type
- Marine Engine Thermostat
- Installation
- Direct replacement
- Operating Temperature
- 55°C (131°F)
- Housing Color
- Silver/metallic
What’s in the Box
- 1 x Thermostat (6D9-12411-00)
Installation Notes
- Ensure the engine is cool and disconnect the battery for safety.
- Locate the thermostat housing on the engine block or cylinder head.
- Place a drain pan beneath the housing and carefully remove the cover bolts to release any remaining coolant.
- Remove the old thermostat. Note its orientation for correct placement of the new part.
- Clean both the housing and cover surfaces thoroughly, removing all old gasket material.
- Install the new thermostat in the correct orientation. A new gasket is highly recommended (not included).
- Re-install the housing cover and tighten the bolts to the manufacturer's recommended torque specification.
- Refill the cooling system with the appropriate coolant.
- Reconnect the battery, start the engine, and carefully check for any leaks around the housing as the engine warms up.
Troubleshooting & FAQ
How do I know if my thermostat has failed?
Common signs include the engine overheating, especially at idle or low speeds, or the engine failing to reach its normal operating temperature. An inconsistent temperature gauge reading can also indicate a faulty thermostat.
Is a new gasket required for installation?
Yes, it is highly recommended to replace the thermostat housing gasket (not included) whenever replacing the thermostat to ensure a proper seal and prevent coolant leaks.
At what temperature does this thermostat open?
This thermostat is calibrated to begin opening at 55°C (131°F) to allow coolant to circulate through the engine.
What happens if I use the wrong temperature thermostat?
Using a thermostat with a temperature rating that is too high can cause engine overheating. A rating that is too low will prevent the engine from reaching its optimal operating temperature, leading to reduced fuel efficiency, increased engine wear, and carbon buildup.
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.
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