Thermostat Kit – Fits Mercury Marine 200-300 HP (Replaces 8M0057307, 885599003)

Product Details

This Thermostat Kit (130°F/54°C) is engineered for a precise fit in a wide range of Mercury Marine 200-300 HP V6 outboard engines. It is compatible with 2.5L, 3.0L, and 3.2L EFI/DFI powerheads, including Pro XS, Sport XS, and XS models, ensuring correct engine operating temperature for optimal performance and efficiency.

It serves as a direct interchange for Mercury OEM part numbers 8M0057307, 885599003, 885599004, and 8855991, as well as Sierra equivalents 18-3606, 18-3535, and 18-3675.

• Designed for direct-fit installation.
• Maintains a consistent 130°F (54°C) operating temperature.
• Kit includes two thermostats and two gaskets for a complete service.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury Marine engines, please verify compatibility using your engine's model name, horsepower, and serial number range against this list.

Technical Specs

Product Type

Thermostat Kit

Installation

Direct fit

Temperature Rating

130°F (54°C)

Application

Outboard engines

Engine Type

6-cylinder

Engine Displacement

2.5L, 3.0L, 3.2L

Weight

3.2–4.0 oz (90–113g)

Package Contents

2 thermostats and 2 gaskets

OEM Supersession

8M0057307 supersedes 885599003, 885599004, 8855991

What’s in the Box

• 2 x Thermostats (Replaces 8M0057307)
• 2 x Thermostat Cover Gaskets (Replaces 27-8503181)

Installation Notes

Perform the following steps with the engine off and cool. Refer to your specific engine's service manual for detailed guidance.

1. Disconnect the battery to prevent accidental starting.
2. Place a drain pan under the engine and drain the cooling system as required.
3. Locate the thermostat housing(s) on the engine block or cylinder heads. Clear any components blocking access.
4. Remove the bolts securing the thermostat cover(s). Note that some coolant may spill.
5. Carefully remove the old thermostat(s) and gasket(s). Take note of their orientation.
6. Thoroughly clean the mating surfaces of the housing and cover, removing all old gasket material and corrosion.
7. Install the new thermostat(s) in the correct orientation, followed by the new gasket(s).
8. Reinstall the cover(s) and tighten the bolts to the manufacturer's specified torque.
9. Refill the cooling system with the appropriate coolant.
10. Reconnect the battery, start the engine, and check for leaks. Allow the engine to reach operating temperature to ensure the thermostats open correctly.

Troubleshooting & FAQ

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.