Thermostat Kit – Fits Mercury/Mariner 4-Stroke 8-40 HP (Replaces 825212A1, 825212001, 855676003)
Thermostat Kit – Fits Mercury/Mariner 4-Stroke 8-40 HP (Replaces 825212A1, 825212001, 855676003) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
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Thermostat Kit – Fits Mercury/Mariner 4-Stroke 8-40 HP (Replaces 82...
Thermostat Kit – Fits Mercury/Mariner 4-Stroke 8-40 HP (Replaces 825212A1, 825212001, 855676003) is backordered and will ship as soon as it is back in stock.
Product Details
This Thermostat Kit – Fits Mercury/Mariner 4-Stroke 8-40 HP Outboard Motors is an essential component for maintaining optimal engine temperature in Mercury/Mariner 4-Stroke 8–40 HP outboard engines from 1995–2018, including specific 30 HP (1999–2005), 40 HP (1998–2018), and 50 HP (1995–2000) models.
It serves as a direct cross-reference for multiple OEM and aftermarket part numbers, including Mercury 825212A1, 825212001, 855676003, 825212T02, 855676A1, and Sierra 18-3642.
- Engineered for direct-fit installation.
- Features a 140°F (60°C) temperature rating.
- Includes the thermostat, rubber seal, and housing gasket for a complete service.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury | 825212A1 |
| Mercury | 825212001 |
| Mercury | 855676003 |
| Mercury | 825212T02 |
| Mercury | 855676A1 |
| Sierra | 18-3642 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For Mercury/Mariner engines, verify your model, horsepower, and serial number range against the data provided to ensure an exact match.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury | 8HP (Bodensee) | 8 | 4-Stroke | 825212A1 | Serial: 0P086824-0P400999 | |
| Mercury | 9.9HP | 9.9 | 4-Stroke | 1995–1998 | 825212A1 | 232cc engine |
| Mercury | 9.9HP | 9.9 | 4-Stroke | 1999–2004 | 825212A1 | 323cc engine, Serial: 0P075166-0P400999 |
| Mercury | 13.5HP (International) | 13.5 | 4-Stroke | 825212A1 | Serial: 0P134638 | |
| Mercury | 15HP | 15 | 4-Stroke | 825212A1 | Serial: 0P9973100 & UP | |
| Mercury | 25HP | 25 | 4-Stroke | 825212A1 | Serial: 0P9973100 & UP | |
| Mercury | 30HP | 30 | 4-Stroke | 1999–2005 | 825212A1 | USA made (0G760300-1B226999), Belgium made (0P017000-0P400999) |
| Mercury | 30HP (Carb) (3 Cyl) (EFI) | 30 | 4-Stroke | 825212A1 | Serial: 0P153500-0P400999 | |
| Mercury | 40HP | 40 | 4-Stroke | 1998–2018 | 825212A1 | Serial: 994408 & UP |
| Mercury | 40HP (Carb) (3 Cyl) | 40 | 4-Stroke | 825212A1 | Serial: 0P153500-0P400999 | |
| Mercury | 40HP (EFI) (3 Cyl) | 40 | 4-Stroke | 825212A1 | Serial: 0P401000-0P400999 | |
| Mercury | 40HP (ITALY) | 40 | 4-Stroke | 825212A1 | Serial: 0P017000-0P094499 | |
| Mercury | 45HP (Bodensee) | 45 | 4-Stroke | 825212A1 | Serial: 9944807 & UP | |
| Mercury | 50HP (BIGFOOT) | 50 | 4-Stroke | 1995–2000 | 825212A1 | Serial: 9944807 & UP, USA made (0G231123-0T178499) |
Technical Specs
- Product Type
- Thermostat Kit
- Installation
- Direct replacement
- Temperature Rating
- 140°F (60°C)
- Fuel Type
- Gasoline
- Mounting Type
- Engine Mount
- Weight
- 0.03 kg (1.13 oz)
- Other Technical Attributes
- For manual operation
What’s in the Box
- 1 x Thermostat (825212A1)
- 1 x Ring Rubber Seal
- 1 x Housing Gasket (27-824853)
- 1 x Installation Instructions
Installation Notes
- Ensure the engine is completely cool and disconnect the negative battery terminal for safety.
- Locate the thermostat housing on the engine. Place a drain pan underneath to catch any spilling coolant.
- Carefully remove the bolts securing the housing cover. Gently tap the cover if needed to break the seal.
- Remove the old thermostat and gaskets. Take note of the thermostat's orientation.
- Thoroughly clean the mating surfaces of the housing and the engine block, removing all old gasket material.
- Install the new thermostat from the kit in the correct orientation.
- Place the new gasket and seal in position and reinstall the housing cover.
- Tighten the bolts evenly to the manufacturer's specified torque to ensure a proper seal.
- Reconnect the battery, refill the cooling system with the appropriate coolant, and check for leaks.
- Start the engine, let it reach operating temperature, and re-check for any leaks.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs include engine overheating, the engine not reaching normal operating temperature (running too cool), erratic temperature gauge readings, or coolant leaks from the thermostat housing area.
Why is there a temperature difference listed (120°F vs. 140°F)?
Different aftermarket sources sometimes report conflicting data. This thermostat is rated at 140°F (60°C), a specification confirmed by multiple retailers for this application. For certainty, always compare with your engine's service manual recommendation.
What is the recommended replacement interval for a thermostat?
Thermostats are a preventative maintenance item. It's wise to inspect them during regular service and consider replacement every 200-300 hours or 2-3 years, especially when operating in saltwater environments.
Is this kit compatible with both Mercury and Mariner branded engines?
Yes, it is designed for a wide range of both Mercury and Mariner 4-stroke outboards within the specified HP and year range. Double-check the compatibility table for your exact model and serial number to confirm fitment.
Do I need to use sealant on the new gaskets?
The included gaskets are typically designed to be installed dry. However, it is best practice to consult your engine's official service manual for specific instructions regarding the use of gasket sealants.
What's the difference between OEM and aftermarket parts?
This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials. Our parts are manufactured in an ISO 9002 facility and are batch-inspected for quality and consistency.
How fast is shipping and what does it cost?
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free to most locations; expedited options and costs are shown at checkout where available.
What is the warranty and return policy?
This part is covered by a 1-year warranty against manufacturing defects under normal use. If you encounter an issue, please contact our support team with your order details. Returns are accepted within our standard window for items that are unused and in original condition. See our policy page or reach out for instructions.
Safety & Important Info
- Verify images and specifications against 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 quality control.
Payment & Security
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