Thermostat (143°F/62°C) – Fits Yamaha 2-Stroke Outboard 9.9-55 HP (Replaces 6F5-12411-01, 6F5-12411-02, 6F5-12411-03)

$23.00 $36.00

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Product Details

This replacement Thermostat (143°F/62°C) – Fits Yamaha 2-Stroke Outboard 9.9-55 HP ensures your engine operates at the correct temperature for optimal performance and longevity. It is engineered for a wide range of Yamaha 2-Stroke models from 9.9 HP to 55 HP, specifically covering years 1986–1997 including series 9.9, 15, C25, C30, C40, C55, CV25, CV30, CV40, and CV55.

This part provides a direct interchange for Yamaha part numbers 6F5-12411-01-00, 6F5-12411-02-00, and 6F5-12411-03-00, as well as Sierra 18-3625.

Designed for direct OEM-replacement fit.
Regulates engine temperature to a consistent 143°F (62°C).
Includes the necessary O-ring/gasket for a complete installation.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Brand	Part Number
Yamaha	6F5-12411-01-00, 6F5-12411-02-00, 6F5-12411-03-00
Sierra	18-3625

Engine & Model Compatibility

Confirm your engine model/year in the table below.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.
Yamaha	9.9	9.9	2-Stroke	1986–1995	6F5-12411-01/02/03
Yamaha	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	9.9MSHP	9.9	2-Stroke	1991	6F5-12411-01/02/03
Yamaha	15	15	2-Stroke	1986–1995	6F5-12411-01/02/03
Yamaha	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	15ML, 15MS	15	2-Stroke	1991–1995	6F5-12411-01/02/03
Yamaha	15MLHP	15	2-Stroke	1991	6F5-12411-01/02/03
Yamaha	C25EL	25	2-Stroke	1991–1997	6F5-12411-01/02/03
Yamaha	C25ML	25	2-Stroke	1993–1997	6F5-12411-01/02/03
Yamaha	C25MS	25	2-Stroke	1992–1997	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	C55TL	55	2-Stroke	1993–1995	6F5-12411-01/02/03
Yamaha	CV55EL	55	2-Stroke	1989	6F5-12411-01/02/03

Technical Specs

Product Type: Thermostat
Installation: Direct OEM Replacement
Operating Temperature: 62°C (143°F)
Materials: High-grade corrosion-resistant housing and spring
Package Contents: Thermostat with O-ring/gasket

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 and disconnect the battery for safety.
Locate the thermostat housing on the engine block or cylinder head. Place a drain pan underneath.
Carefully remove the bolts securing the housing cover. A small amount of coolant may spill.
Note the orientation of the old thermostat before removing it. Remove the old gasket and clean both mating surfaces thoroughly.
Install the new thermostat in the same orientation as the original. Place the new gasket onto the housing.
Re-secure the housing cover, tightening the bolts to the manufacturer-specified torque in a crisscross pattern.
Reconnect the battery, refill the cooling system, and run the engine while checking for leaks.

Troubleshooting & FAQ

What are the signs of a bad thermostat?

A faulty thermostat can cause the engine to overheat (if stuck closed) or run too cool (if stuck open), leading to poor performance, increased fuel consumption, and potential engine damage. An erratic temperature gauge is another common indicator.

How often should I replace my outboard's thermostat?

For preventative maintenance, it's wise to inspect and consider replacing your thermostat every 200-300 hours or every 2-3 years, especially if operating in saltwater. Always consult your engine's service manual for specific recommendations.

Is this the correct temperature rating for my engine?

This thermostat is rated for 143°F (62°C), which is the standard OEM specification for the listed Yamaha models. Using the correct temperature thermostat is critical for engine efficiency and longevity.

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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