Thermostat (60°C/140°F) – Fits Mercury/Mariner 75-225 HP (Replaces 8M0175321, 855676004, 8M6010296)
Thermostat (60°C/140°F) – Fits Mercury/Mariner 75-225 HP (Replaces 8M0175321, 855676004, 8M6010296) 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 (60°C/140°F) – Fits Mercury/Mariner 75-225 HP (Replaces ...
Thermostat (60°C/140°F) – Fits Mercury/Mariner 75-225 HP (Replaces 8M0175321, 855676004, 8M6010296) is backordered and will ship as soon as it is back in stock.
Product Details
This Thermostat (60°C/140°F) – Fits Mercury/Mariner 75-225 HP Outboard Engines (Replaces 8M0175321, 855676004, 8M6010296) is a crucial component for maintaining optimal engine temperature in Mercury and Mariner outboard engines. It is designed for a precise fit on 75 HP, 80 HP, 90 HP, 100 HP, and 225 HP 4-stroke models across various serial number ranges.
It serves as a direct interchange for Mercury part numbers 855676004, 8M0175321, 8M6010296, and 8556761, as well as Sierra 18-3632.
- Engineered for direct OEM-replacement fit and function.
- Regulates engine temperature at a 60°C (140°F) opening threshold.
- Specifically designed for use in closed cooling systems on outboard marine engines.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury | 855676004 |
| Mercury | 8M0175321 |
| Mercury | 8M6010296 |
| Mercury | 8556761 |
| Sierra | 18-3632 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This thermostat is compatible with a range of 4-stroke outboard models from Mercury and Mariner, including specific CARB and EFI versions based on serial number.
| Brand | Model | Rated HP | Stroke | OEM Part No. | Notes |
|---|---|---|---|---|---|
| Mercury | Outboard | 75 | 4-Stroke | 855676004/8M0175321/8M6010296 | CARB models (Serial 0G960500 through 1B226999) |
| Mercury | Outboard | 75 | 4-Stroke | 855676004/8M0175321/8M6010296 | EFI models (Serial 0G1B172291 through 1B366822) |
| Mercury | Outboard | 80 | 4-Stroke | 855676004/8M0175321/8M6010296 | EFI models (Serial 1B227000 through 1B366822) |
| Mercury | Outboard | 90 | 4-Stroke | 855676004/8M0175321/8M6010296 | CARB models (Serial 0G960500 & Up) |
| Mercury | Outboard | 90 | 4-Stroke | 855676004/8M0175321/8M6010296 | EFI models (Serial 1B172291 through 1B366822) |
| Mercury | Outboard | 100 | 4-Stroke | 855676004/8M0175321/8M6010296 | EFI models (Serial 1B227000 through 1B366822) |
| Mercury | Outboard | 225 | 4-Stroke | 855676004/8M0175321/8M6010296 | EFI models (Serial 0T653945 & Up) |
Technical Specs
- Product Type
- Marine Engine Thermostat
- Installation
- Direct OEM replacement
- Temperature Rating
- 60°C (140°F)
- Housing Material
- Brass/Stainless Steel
- Operating Temperature
- 60°C (140°F) opening temperature
- Other Technical Attributes
- Designed for closed cooling systems in outboard marine engines
What’s in the Box
- 1 x 60°C (140°F) Thermostat (855676004/8M0175321/8M6010296)
Installation Notes
- Ensure the engine has cooled down completely before beginning work.
- Drain the engine coolant to a level below the thermostat housing.
- Carefully remove the bolts securing the thermostat housing cover.
- Note the orientation of the old thermostat before lifting it out.
- Remove the old thermostat and any existing gasket or seal material. Thoroughly clean the mating surfaces of the housing and cover.
- Install the new thermostat in the correct orientation.
- Place the new gasket or seal (not included with this part) and reinstall the housing cover, tightening bolts to the manufacturer's specifications.
- Refill the cooling system with the correct type and amount of coolant.
- Start the engine, check for any leaks, and monitor the temperature gauge to confirm the thermostat opens at the correct temperature.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs of a faulty thermostat include engine overheating (stuck closed) or taking too long to reach operating temperature (stuck open). You may see alarms on your dash, erratic temperature gauge readings, or notice increased fuel consumption.
Why is the 60°C (140°F) temperature rating important?
This rating ensures the engine warms up quickly but doesn't overheat. It's the temperature at which the thermostat begins to open, allowing coolant to circulate through the engine. Using the wrong temperature can lead to poor performance, carbon buildup, or engine damage.
Can I test my old thermostat?
Yes. Suspend the thermostat in a pot of water with a thermometer. Heat the water and observe the temperature at which the thermostat begins to open. If it doesn't open at or near its 60°C (140°F) rating, or if it doesn't open at all, it should be replaced.
Is a gasket or seal included with this thermostat?
This listing is for the thermostat only. Gaskets and seals are sold separately. It is critical to use new gaskets/seals during installation to prevent coolant leaks.
How often should a marine thermostat be replaced?
Thermostats don't have a fixed replacement interval, but it's good practice to inspect them during regular cooling system service (every 1-2 years for saltwater use) and replace them if there are any signs of corrosion or if engine temperature issues arise.
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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