Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP (Replaces 27-850318, 27-8503181)
Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP (Replaces 27-850318, 27-8503181) 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.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
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Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP (Replaces ...
Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP (Replaces 27-850318, 27-8503181) is backordered and will ship as soon as it is back in stock.
Product Details
This Thermostat Cover Gasket – Fits Mercury Marine 200-300 HP Outboard Motors ensures a proper seal for your engine's cooling system. It is designed for a wide range of Mercury Marine V6 outboard engines from 200 HP to 300 HP, including 2.5L and 3.0L models, as well as 250 HP SportJet DFI Jet Drives.
This part directly replaces Mercury Marine part numbers 27-850318 and 27-8503181, Sierra 18-3675, and GLM 516-39, providing a reliable alternative for maintenance and repair.
- Engineered for direct-fit installation.
- Resistant to heat, oil, and coolant in marine environments.
- Restores seal integrity to prevent coolant leaks.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number | Superseded By |
|---|---|---|
| Mercury Marine | 27-850318 | 27-8503181 |
| Sierra | 18-3675 | |
| GLM | 516-39 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
Fitment for: Mercury Marine & SportJet
| Brand | Model | Rated HP | Stroke | OEM Part No. | Notes |
|---|---|---|---|---|---|
| Mercury Marine | 200 (3.0L EFI) | 200 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 200 HP 6 CYL (2.5L) | 200 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 200 HP 6 CYL (3.0L) | 200 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 200 HP DTS 6 CYL (3.0L) | 200 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 200 HP Pro XS 6 CYL (3.0L) | 200 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 (3.0L EFI) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 (3.0L EFI) MARATHON | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 (3.0L EFI) SEAPRO | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP 6 CYL (2.5L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP 6 CYL (3.0L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP DTS 6 CYL (3.0L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP Pro XS 6 CYL (3.0L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP SPORT XS 6 CYL (2.5L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP SPORT XS 6 CYL (3.0L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 225 HP XS 6 CYL (2.5L) | 225 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 250 (3.0L EFI) | 250 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 250 DFI JET DRIVE (POWERHEAD) | 250 | 27-850318, 27-8503181 | SportJet | |
| Mercury Marine | 250 DFI JET DRIVE (PUMP) | 250 | 27-850318, 27-8503181 | SportJet | |
| Mercury Marine | 250 DFI M2 JET DRIVE (POWERHEAD) | 250 | 27-850318, 27-8503181 | SportJet | |
| Mercury Marine | 250 DFI M2 JET DRIVE (PUMP) | 250 | 27-850318, 27-8503181 | SportJet | |
| Mercury Marine | 250 HP 6 CYL (3.0L) | 250 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 250 HP Pro XS 6 CYL (3.0L) | 250 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 250 HP Sport XS 6 CYL (2.5L) | 250 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 250 HP XS 6 CYL (3.0L) | 250 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 250 LONG (3.0L)(EFI) | 250 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 3.0L EFI MARATHON | 4-Stroke | 27-850318, 27-8503181 | ||
| Mercury Marine | 3.0L EFI SEAPRO | 4-Stroke | 27-850318, 27-8503181 | ||
| Mercury Marine | 300 HP XS 6 CYL (3.2L) | 300 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | 300X (3.0L EFI) PRO MAX | 300 | 4-Stroke | 27-850318, 27-8503181 | |
| Mercury Marine | JP 3.0L DFI | 27-850318, 27-8503181 | SportJet |
Technical Specs
- Product Type
- Thermostat Cover Gasket
- Installation
- Direct replacement
- Material
- Composite/Rubber
- Application
- Marine engines
- Engine Type
- Outboard, SportJet
- Engine Size
- 2.5L, 3.0L, 3.2L V6
- Color
- Grey/Black
- Warranty
- Varies by manufacturer (1 year typical)
What’s in the Box
- 1 x Thermostat Cover Gasket (27-850318/27-8503181)
Installation Notes
- Ensure the engine is off and completely cool before starting.
- Disconnect the battery to prevent accidental starting.
- Locate the thermostat housing on the engine block.
- Place a drain pan under the housing to catch any residual coolant.
- Carefully remove the bolts securing the thermostat cover.
- Remove the old cover and thermostat. Note the thermostat's orientation.
- Thoroughly clean both the cover and the engine block mating surfaces. Remove all old gasket material and corrosion.
- Place the new thermostat into the housing in the correct orientation.
- Position the new gasket on the clean housing surface.
- Reinstall the cover and tighten the bolts in a crossing pattern to the manufacturer's specified torque. Do not overtighten.
- Reconnect the battery, refill the cooling system, and check for leaks after starting the engine.
Troubleshooting & FAQ
How do I know if my thermostat gasket is failing?
Signs of a failing thermostat gasket include visible coolant leaks around the thermostat housing, engine overheating alarms, or an unusually low coolant level. A crusty, white or greenish residue may also be present on the outside of the housing.
Can I reuse my old gasket?
No. Thermostat gaskets are single-use components designed to crush and seal. Reusing a gasket will almost certainly result in a leak and potential engine damage from overheating.
Is this gasket resistant to saltwater?
Yes, this gasket is made from a high-quality composite/rubber material designed to withstand the harsh conditions of both freshwater and saltwater marine environments.
What is the recommended replacement interval?
It's good practice to replace the thermostat and its gasket during routine cooling system service, typically every 2-3 years or 200-300 hours, or whenever the thermostat itself is replaced.
Is this an OEM part or aftermarket?
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.
What are the shipping speed and cost?
We ship worldwide quickly—typical transit time is 5–15 days. Standard shipping is free for most orders, and expedited options will appear at checkout if they are available for your location.
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 any issues, please contact our support team with your order details. Returns are accepted within our standard window, provided the item is unused and in its 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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