Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder Outboards (Replaces 825212A1, 825212001, 855676A1)

Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder Outboards

This thermostat kit is designed for Mercury/Mariner 25–40 HP, 3-cylinder, 4-Stroke outboards from 1995–2018. It serves as a reliable replacement for multiple OEM and aftermarket parts to maintain optimal engine operating temperature.

It directly replaces Mercury parts 825212A1, 825212001, and 855676A1; Sierra part 18-3642; and Yamaha parts 62Y-12414-00/01, 65W-12414-00, and 6H3-12414-00/A1.

• Complete Kit: Includes the thermostat, thermostat cover gasket, and seal for a full replacement.
• Operating Temperature: Engineered to open at 140°F (60°C) to regulate engine temperature effectively.
• Simple Installation: Designed for direct replacement using common tools.

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.

Technical Specifications

Package Contents

• 1 x Thermostat (825212)
• 1 x Thermostat cover gasket (824853)
• 1 x Thermostat seal

Installation Guide

1. Ensure the engine is completely cool and disconnect the battery cables for safety.
2. Locate the thermostat housing on the engine block. Place a drain pan underneath to catch any spilling coolant.
3. Carefully remove the bolts that secure the thermostat housing cover.
4. Take out the old thermostat and its gasket. Pay close attention to the orientation of the thermostat as you remove it.
5. Thoroughly clean the mating surfaces of the housing and its cover, ensuring all old gasket material is removed.
6. Install the new thermostat seal and gasket into the housing.
7. Place the new thermostat into the housing, making sure it is oriented correctly.
8. Re-attach the housing cover and tighten the bolts to the manufacturer's specified torque.
9. Reconnect the battery.
10. Refill the cooling system with the appropriate coolant.
11. Start the engine and let it run for several minutes, carefully checking for any leaks around the thermostat housing.

Troubleshooting & FAQ

Q: How do I confirm this thermostat kit fits my engine?
A: Cross-reference your engine's serial number with the "Compatible Engine Models" table on this page. For Mercury owners, we provide a link to a serial number guide to help you identify your specific model and year.

Q: What is the operating temperature of this thermostat?
A: This thermostat is rated to begin opening at 140°F (60°C). Please note that some OEM parts may be rated for 120°F. This is a 140°F aftermarket unit, so always confirm the correct temperature specification for your engine before installation.

Q: What are the signs of a faulty thermostat?
A: Common symptoms of a failing thermostat include the engine overheating, the engine running too cool and failing to reach its proper operating temperature, or erratic readings on the temperature gauge.

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.