Thermostat (60°C/140°F) – Fits Yamaha 4-Stroke 2.5-100 HP (Replaces 66M-12411-01, 6G8-12411-00)

Thermostat (60°C/140°F) – Fits Yamaha 4-Stroke 2.5-100 HP & Honda 8-225 HP

This thermostat is engineered to maintain optimal engine temperature for a wide range of Yamaha and Honda 4-stroke outboard motors. It ensures your engine operates within its ideal temperature range, preventing both overheating and overcooling to maximize performance and longevity.

This component is a direct replacement for multiple OEM part numbers, fitting Yamaha models from 2.5 to 100 HP and Honda models from 8 to 225 HP. Please see the cross-reference and compatibility tables below to verify fitment for your specific engine.

• Operating Temperature: Opens at 60°C (140°F)
• Package Contents: Includes 1x thermostat and 1x thermostat seal for a complete installation.
• Installation: Designed for direct replacement of the original unit.

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.

Confirm your engine model/year in the table below.

Technical Specifications

Package Contents

• 1 x Thermostat (66M-12411-00/66M-12411-01)
• 1 x Thermostat Seal (688-12412)

Installation Guide

1. Safety First: Ensure the engine is completely cool and disconnect the battery.
2. Locate Housing: Find the thermostat housing on the engine block or cylinder head.
3. Drain Coolant: Place a container underneath and carefully drain a sufficient amount of coolant.
4. Remove Housing: Unbolt and remove the thermostat housing cover. Note the orientation of the old thermostat.
5. Remove Old Parts: Take out the old thermostat and its gasket or seal.
6. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and engine block.
7. Install New Parts: Insert the new thermostat in the correct orientation, followed by the new thermostat seal.
8. Reassemble: Re-install the housing cover and tighten the bolts to the manufacturer's specified torque.
9. Refill Coolant: Top off the cooling system with the appropriate coolant.
10. Final Check: Reconnect the battery, start the engine, and carefully check for any leaks as it warms up.

Troubleshooting & FAQ

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

A: First, check the "Part Number Cross-Reference" table to see if your old part number is listed. Second, find your engine in the "Compatible Engine Models" table. For Yamaha owners, the provided serial number and parts diagram guides can help you verify the exact part needed for your motor.

Q: What are the common signs of a failing thermostat?

A: A thermostat can fail by sticking open or closed. If it sticks open, your engine may not reach its proper operating temperature, leading to poor performance and fuel economy. If it sticks closed, the engine will overheat, which can cause severe damage. An erratic temperature gauge is another common symptom.

Q: What temperature does this thermostat open at?

A: This thermostat is rated to begin opening at 60°C (140°F) to regulate engine temperature.

Q: Is this an OEM part?

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.