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

Thermostat (143°F/62°C) – Fits Yamaha 2-Stroke Outboard 9.9-55 HP

This thermostat is engineered for Yamaha 2-Stroke outboard engines from 9.9 to 55 HP, covering models manufactured between 1986 and 1997. It ensures your engine maintains the correct operating temperature for optimal performance and longevity.

It serves as a direct replacement for Yamaha part numbers 6F5-12411-01-00, 6F5-12411-02-00, 6F5-12411-03-00, and Sierra part number 18-3625.

• Regulates engine temperature at 143°F (62°C).
• Designed as a direct-fit OEM replacement for easy installation.
• Package includes the thermostat and a new O-ring/gasket.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Yamaha owners, here’s a guide to help you identify your engine’s serial/model: Yamaha Serial Number Guide. You can also review exploded diagrams to verify part numbers: Yamaha Parts Diagrams & Catalog.

Technical Specifications

Package Contents

• 1 x Thermostat (6F5-12411-03-00)
• 1 x O-ring/Gasket

Installation Guide

1. Ensure the engine is completely cool before beginning. Disconnect the battery for safety.
2. Locate the thermostat housing on the engine block, referring to your service manual if needed.
3. Place a drain pan under the housing and remove the bolts holding the cover in place. Expect some coolant to spill.
4. Carefully remove the old thermostat and the old gasket or O-ring. Note the orientation of the thermostat.
5. Clean the mating surfaces of the housing and cover, removing any old gasket material.
6. Install the new thermostat in the correct orientation, followed by the new gasket/O-ring.
7. Reattach the housing cover and tighten the bolts to the manufacturer's specified torque. Do not overtighten.
8. Reconnect the battery, refill the cooling system, and check for leaks after starting the engine.

Troubleshooting & FAQ

Q: How do I confirm this thermostat fits my engine?

A: The most reliable method is to match the part number (6F5-12411-01, 6F5-12411-02, or 6F5-12411-03) with your existing part or service manual. You can also use our compatibility table to check your engine model and year range (Yamaha 2-Stroke 9.9-55 HP, 1986-1997).

Q: What are the signs of a failing thermostat?

A: Common symptoms include engine overheating (stuck closed) or the engine failing to reach normal operating temperature (stuck open). An audible alarm or a warning on your gauge is a primary indicator. Also a rough idle could be a symptom.

Q: Is this the correct temperature rating for my engine?

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

Q: OEM vs. Aftermarket?

A: This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

Q: How fast do you ship and how much does it cost?

A: We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

Q: What is your warranty and return policy?

A: This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.