Thermostat Gasket for Mercury/Mariner 200–250 HP 3.0L DFI and EFI Outboards – Replaces 27-850318 and 27-8503181
Thermostat Gasket for Mercury/Mariner 200–250 HP 3.0L DFI and EFI Outboards – Replaces 27-850318 and 27-8503181 is backordered and will ship as soon as it is back in stock.
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Thermostat Gasket for Mercury/Mariner 200–250 HP 3.0L DFI and EFI O...
Thermostat Gasket for Mercury/Mariner 200–250 HP 3.0L DFI and EFI Outboards – Replaces 27-850318 and 27-8503181 is backordered and will ship as soon as it is back in stock.
Product Details
This thermostat gasket is designed to seal the thermostat housing on specified Mercury and Mariner outboard engines. Catalog fitment identifies this gasket for 200–250 HP 3.0L DFI and EFI outboards.
It replaces Mercury Marine part numbers 27-850318 and 27-8503181. Sierra catalog reference 18-3675 is also documented for the same replacement numbers.
- Replacement gasket for the thermostat housing.
- Helps restore the seal between the thermostat housing and its mating surface.
- Suitable for maintenance or repair when the original gasket is removed.
- Confirm the exact engine model, year, serial number, and original gasket before ordering.
Part Number Matrix
The following references are documented as replacement or equivalent catalog numbers for this gasket.
| Brand | Part Number | Reference Type |
|---|---|---|
| Mercury Marine | 27-850318 | OEM replacement reference |
| Mercury Marine | 27-8503181 | OEM replacement reference |
| Sierra | 18-3675 | Aftermarket catalog reference |
Engine & Model Compatibility
Available catalog evidence identifies the following application scope. Fitment should still be confirmed against the engine identification label and the gasket removed from the engine.
| Brand | Application | Rated HP | Displacement | Fuel System | Replacement References |
|---|---|---|---|---|---|
| Mercury / Mariner | Specified outboard applications | 200–250 HP | 3.0L | DFI and EFI | 27-850318; 27-8503181 |
Technical Specs
- Product Type
- Thermostat gasket
- Application
- Mercury / Mariner outboard thermostat housing
- Documented Engine Range
- 200–250 HP, 3.0L, DFI and EFI
- Installation
- Replacement gasket; direct-fit application where the listed reference matches
- OEM References
- 27-850318 and 27-8503181
- Aftermarket Reference
- Sierra 18-3675
What’s in the Box
- 1 x Thermostat gasket
Installation Notes
- Switch off the engine and allow it to cool completely before beginning work.
- Disconnect the battery or follow the engine manufacturer's safe service procedure to prevent accidental starting.
- Locate the thermostat housing and place a suitable drain pan below the service area.
- Remove the thermostat housing cover carefully, retaining the hardware for reinstallation.
- Remove the old gasket and clean the mating surfaces thoroughly. Do not scratch or deform the housing surfaces.
- Inspect the thermostat, housing, and fasteners for damage or corrosion. Replace damaged components as required.
- Position the new gasket correctly on the clean mating surface.
- Reinstall the thermostat housing cover and tighten the fasteners to the manufacturer's specified torque and sequence.
- Restore the cooling-water supply or operating conditions, start the engine according to the manufacturer's procedure, and check the housing area for leaks.
Troubleshooting & FAQ
What engines does this gasket fit?
The documented application is Mercury/Mariner 200–250 HP 3.0L outboards using DFI or EFI systems. Because model and serial-number ranges are not fully specified in the available record, verify the exact engine identification and original gasket before ordering.
What part numbers does this gasket replace?
This gasket is listed as replacing Mercury Marine part numbers 27-850318 and 27-8503181. Sierra reference 18-3675 is the corresponding aftermarket catalog number.
Can I reuse the old thermostat gasket?
Replacement is recommended whenever the housing is opened. Reusing a compressed, damaged, or contaminated gasket can prevent a reliable seal and may result in cooling-system leakage.
What should I check before purchasing?
Compare the original gasket and housing, then confirm the engine brand, horsepower, displacement, fuel-system type, model year, and serial number. Do not rely on horsepower alone when confirming fitment.
Is this an OEM part?
This listing is for an aftermarket replacement gasket. The referenced Mercury Marine numbers are OEM replacement references, not a claim that the listed gasket is manufactured by Mercury Marine.
Safety & Important Info
- Work only on a cool, stopped engine and follow the engine manufacturer's service instructions.
- Verify the gasket shape and all fitment details against your original part before installation.
- Incorrect thermostat-housing sealing can cause cooling-water leakage or engine overheating.
- Professional installation is recommended if you are not familiar with outboard cooling-system service.
About Sea Sierra
- Marine Replacement Parts — We supply replacement components for routine maintenance and repair of marine engines.
- Fitment-Focused Listings — Product references and application details are provided to help customers compare parts before ordering.
- Service Support — Use your engine identification information and original part details when confirming compatibility.
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