Thermostat (60°C/140°F) – Fits Honda, Mercury & Yamaha 8-255 HP (Replaces 19300-ZW9-003, 6G8-12411-00)

Thermostat (60°C/140°F) – Fits Honda, Mercury & Yamaha 8-255 HP

This aftermarket thermostat is designed for a wide range of Honda, Mercury, and Yamaha 4-stroke outboard engines. It ensures your engine operates at the correct temperature for optimal performance and longevity by opening at 60°C (140°F).

Engineered as a direct replacement for Honda parts 19300-ZW9-003 and 19300-ZY3-023; Mercury parts including 825212 and 855676; and Yamaha parts such as 6G8-12411-00-00 and 66M-12411-01-00.

• Operating Temperature: 60°C / 140°F
• Installation: Direct replacement for OEM parts.
• Construction: Built with an OEM-grade brass and rubber composite for reliable service.

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 Mercury owners, here’s a guide to help you identify your engine’s serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

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.

Honda

Mercury

Yamaha

Technical Specifications

Package Contents

• 1 x Thermostat Assembly (60°C/140°F rating)

Installation Guide

1. Safety First: Ensure the engine is completely cool and disconnect the battery before starting.
2. Locate Housing: Find the thermostat housing on the engine block, typically where the main coolant hose connects.
3. Drain Coolant: Place a container underneath and drain a sufficient amount of coolant to bring the level below the thermostat.
4. Remove Housing: Loosen and remove the bolts securing the thermostat housing cover.
5. Extract Old Thermostat: Carefully remove the old thermostat and any gasket or sealing ring material. Note its orientation.
6. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and engine block to ensure a proper seal.
7. Install New Thermostat: Place the new thermostat into the housing in the correct orientation, along with its new gasket (if included).
8. Secure Housing: Re-attach the housing cover and tighten the bolts to the manufacturer's recommended torque specification.
9. Refill Coolant: Top off the cooling system with the appropriate coolant.
10. Check for Leaks: Reconnect the battery, start the engine, and let it run for a few minutes while carefully checking for any leaks around the housing.

Troubleshooting & FAQ

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

A: The best way is to compare your original part number to the list in our "Part Number Cross-Reference" table. Additionally, check the "Compatible Engine Models" tables to see if your engine's make, model, HP, and year range are listed.

Q: What are the signs of a failing thermostat?

A: Common symptoms include engine overheating (stuck closed) or taking too long to reach operating temperature (stuck open). You may see a high reading on your temperature gauge or receive an engine alarm.

Q: Is a gasket included with this thermostat?

A: This listing is for the thermostat assembly as pictured. Gaskets are often specific to the housing and may need to be purchased separately if not included or if your original is damaged.

Q: What temperature does this thermostat operate at?

A: This thermostat begins to open at 60°C (140°F) to regulate engine temperature.

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.