Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder (Replaces 825212A1, 825212001, 855676A1)
Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder (Replaces 825212A1, 825212001, 855676A1) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
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Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder (Replaces...
Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder (Replaces 825212A1, 825212001, 855676A1) is backordered and will ship as soon as it is back in stock.
Product Details
This Thermostat Kit – Fits Mercury/Mariner 25-40 HP 3-Cylinder Outboards (Replaces 825212A1, 825212001, 855676A1) is an essential component for maintaining correct engine operating temperature. It is designed specifically for Mercury and Mariner 25–40 HP, 3-cylinder, 4-stroke outboards manufactured between 1995 and 2018.
It serves as a direct interchange for multiple part numbers, including Mercury 825212A1, 825212001, and 855676A1; Sierra 18-3642; GHmarine 18-3642; and Yamaha 62Y-12414-00/01, 65W-12414-00, and 6H3-12414-00/A1.
- Engineered for a precise 140°F (60°C) opening temperature.
- Allows for direct replacement using common tools.
- Includes thermostat, gasket, and seal for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury | 825212A1 |
| Mercury | 825212001 |
| Mercury | 855676A1 |
| Sierra | 18-3642 |
| GHmarine | 18-3642 |
| Yamaha | 62Y-12414-00 |
| Yamaha | 62Y-12414-01 |
| Yamaha | 65W-12414-00 |
| Yamaha | 6H3-12414-00 |
| Yamaha | 6H3-12414-A1 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For Mercury and Mariner models, verify your engine's serial number or year range against the chart to ensure correct fitment.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury | 4-Stroke | 25 | 4-Stroke | 0T409000-1B344298 | 825212A1 | |
| Mercury | 4-Stroke Carb | 30 | 4-Stroke | 0T409000-1B226999 | 825212A1 | |
| Mercury | 4-Stroke EFI | 30 | 4-Stroke | 0T409000-1B226999 | 825212A1 | |
| Mercury | 4-Stroke EFI | 30 | 4-Stroke | 1C109093-1C456299 | 825212A1 | |
| Mercury | 4-Stroke EFI | 30 | 4-Stroke | 1C456300-UP | 825212A1 | |
| Mercury | 4-Stroke Carb | 40 | 4-Stroke | 0T409000-1B226999 | 825212A1 | |
| Mercury | 4-Stroke Carb | 40 | 4-Stroke | 1B227000-1B344306 | 825212A1 | |
| Mercury | 4-Stroke Carb | 40 | 4-Stroke | 1C000001-1C018825 | 825212A1 | |
| Mercury | 4-Stroke Carb | 40 | 4-Stroke | 1C018826-UP | 825212A1 | |
| Mercury | 4-Stroke EFI (3 Cyl) | 40 | 4-Stroke | 0T409000-1B226999 | 825212A1 | |
| Mercury | 4-Stroke EFI (3 Cyl) | 40 | 4-Stroke | 1B227000-1B344398 | 825212A1 | |
| Mercury | 4-Stroke EFI (3 Cyl) | 40 | 4-Stroke | 1C000001-1C043416 | 825212A1 | |
| Mercury | 4-Stroke EFI (3 Cyl) | 40 | 4-Stroke | 1C456300-UP | 825212A1 | |
| Mercury | 4-Stroke EFI (3 Cyl) | 40 | 4-Stroke | 1C043417-1C456299 | 825212A1 | |
| Mercury | FourStroke | 30 | 4-Stroke | 1999–2005 (USA 0G760300-1B226999) | 825212A1 | Alternate info source |
| Mercury | FourStroke | 30 | 4-Stroke | 1999–2005 (Belgium 0P017000-0P400999) | 825212A1 | Alternate info source |
| Mercury | FourStroke | 40 | 4-Stroke | 1998–2018 (USA 0G231123 & up) | 825212A1 | Alternate info source |
| Mercury | FourStroke | 40 | 4-Stroke | 1999–2018 (Belgium 0P017000 & up) | 825212A1 | Alternate info source |
| Mercury | FourStroke | 50 | 4-Stroke | 1995–2000 (USA 0G231123-0T178499) | 825212A1 | Alternate info source |
Technical Specs
- Product Type
- Thermostat Kit
- Installation
- Direct replacement (removal and installation with common tools)
- Temperature Rating
- 140°F (60°C)
- Housing Material
- Metal
- Operating Temperature
- 140°F (60°C) opening temperature
- Included Components
- Gasket and seal
What’s in the Box
- 1 x Thermostat (825212)
- 1 x Thermostat cover gasket (824853)
- 1 x Thermostat seal
Installation Notes
- Ensure the engine is completely cool before starting.
- Disconnect the negative battery cable for safety.
- Locate the thermostat housing on the engine. Place a drain pan underneath.
- Slowly loosen the housing cover bolts to drain the coolant.
- Remove the housing cover, old thermostat, and old gasket or seal.
- Thoroughly clean the mating surfaces of the housing and cover, removing all old gasket material.
- Install the new thermostat, ensuring it is oriented correctly.
- Place the new gasket and seal in position and reinstall the housing cover.
- Tighten the bolts to the manufacturer’s recommended torque specification.
- Refill the cooling system with the appropriate coolant, reconnect the battery, and start the engine.
- Check for any leaks and monitor the engine temperature to ensure proper operation.
Troubleshooting & FAQ
Is this thermostat rated for 120°F or 140°F?
This specific kit contains a thermostat with a 140°F (60°C) opening temperature. While some OEM or aftermarket parts for other applications might be 120°F, this model is designed for the 140°F specification. Always confirm the required temperature for your engine before purchasing.
What are the signs of a faulty thermostat?
Common symptoms include engine overheating, the engine not reaching normal operating temperature (running too cool), erratic temperature gauge readings, and coolant leaks appearing around the thermostat housing.
Will this fit my 50 HP Mercury outboard?
This kit is documented primarily for 25-40 HP 3-cylinder models. While some data suggests compatibility with specific 50 HP FourStroke models (USA 0G231123-0T178499), it is critical to verify the required OEM part number (825212A1) for your engine's serial number before ordering.
How often should an outboard thermostat be replaced?
As a preventative measure, it is good practice to inspect the thermostat annually and replace it every 2-3 years, or during routine cooling system service. Replacement intervals may be shorter for boats operated primarily in saltwater.
Can I install this myself?
Yes, this is a common DIY task for boat owners with basic mechanical skills. You will need common hand tools, a torque wrench, and a new container of appropriate coolant. Always follow the steps in your engine’s service manual for specific instructions and torque values.
What if my engine still overheats after replacement?
If overheating persists, the issue may lie elsewhere in the cooling system. Check for a worn water pump impeller, blockages in cooling water passages, restrictions in hoses, or an air lock in the system.
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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