Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP Outboard Motors (Replaces 27-850318, 27-8503181)

Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP Outboard Motors (Replaces 27-850318, 27-8503181)

This thermostat cover gasket is designed for a precise fit on Mercury Marine 200-300 HP 3.0L and 2.5L V6 outboard engines, as well as 250 HP SportJet DFI Jet Drives. It ensures a reliable seal for the thermostat housing, preventing coolant leaks and maintaining optimal engine operating temperature.

It serves as a direct replacement for several OEM and aftermarket part numbers, offering a reliable and cost-effective solution for maintenance and repair.

• Installation Type: Direct replacement
• Material: High-quality composite/rubber blend for durability and a secure seal
• Package Includes: One thermostat cover gasket

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 Cover Gasket (Replaces 27-850318 / 27-8503181)

Installation Guide

Professional installation is recommended. Always consult your engine's service manual for detailed instructions and torque specifications.

1. Safety First: Ensure the engine is cool and disconnect the battery cables before starting.
2. Drain Coolant: Partially drain the engine's cooling system to a level below the thermostat housing.
3. Locate Housing: Identify the thermostat housing on the engine block.
4. Remove Cover: Loosen and remove the bolts securing the thermostat cover. Keep track of bolt lengths if they differ.
5. Remove Old Gasket: Carefully remove the old thermostat cover and gasket. Note the orientation of the thermostat if you are also replacing it.
6. Clean Surfaces: Thoroughly clean both the housing and cover mating surfaces. Remove all old gasket material and corrosion to ensure a proper seal.
7. Install New Gasket: Place the new gasket onto the thermostat housing, ensuring it is correctly aligned with the bolt holes.
8. Reassemble: Reinstall the thermostat cover and tighten the bolts in a cross-pattern to the manufacturer's recommended torque specification. Do not overtighten.
9. Refill and Check: Refill the cooling system with the appropriate coolant. Start the engine and let it run, carefully checking for any leaks around the gasket.

Troubleshooting & FAQ

Q: How do I confirm this gasket fits my engine?
A: Start by comparing your existing part to our product images. Then, check the Part Number Cross-Reference table to see if your old part number is listed. Finally, review the Compatible Engine Models table to ensure your specific engine model is included.

Q: What are the common signs of a failing thermostat cover gasket?
A: The most common signs are coolant leaks originating from the thermostat housing area, which can lead to engine overheating. You may notice dried coolant residue (typically green, pink, or yellow) on the engine or smell a sweet "hot maple syrup" odor after the engine has been running.

Q: How often should I replace this gasket?
A: It is best practice to replace the thermostat cover gasket anytime the thermostat itself is replaced. You should also replace it immediately if you notice any signs of leaking or material degradation during routine inspections.

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.