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Mercury Outboard Parts
Shop Mercury outboard motor parts by system, including cooling, fuel delivery, electrical, internal engine components, lower unit parts, and routine service items. Browse OEM and replacement Mercury marine parts to support maintenance, repair, and rebuild projects.
Shop by System
- Complete Powerheads
- Control Cables & Boxes
- Cooling System
- Cowlings & Components
- Electrical System
- Engine Oil
- Engine Rebuild Kits
- Fuel & Carburetion System
- Intake System
- Internal Engine Components
- Lower Unit Components
- Oiling System
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- Service & Oil Change Kits
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- Transom & Midsection
- Trim System
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Common Mercury Outboard Model References
This category supports many popular Mercury outboards across a wide horsepower range. Always confirm your model and serial number for best fitment.
- Portable & Midrange: 2.5HP–60HP (2-stroke & FourStroke)
- Performance: Pro XS and EFI platforms (75HP–300HP)
- Direct Injection: Optimax models (75HP–250HP)
- Premium Offshore: Verado models (135HP–400HP)
- Commercial: SeaPro models (commonly 75HP–300HP)
Always confirm your exact horsepower, model year, and serial number to ensure proper part compatibility before ordering.
878231T2
OEM Mercury Thermostat Cover Kit
Mercury 3.0L EFI / DFI / OptiMax Thermostat Cover Assembly
This OEM Mercury thermostat cover kit is designed for select Mercury and Mariner 3.0L EFI, DFI, OptiMax, and Pro XS V6 outboards.
This cover kit helps restore proper thermostat sealing and cooling system performance on compatible Mercury 3.0L high-performance outboards.
Compatible Applications
| Horsepower Range | Engine Family | Engine Type | Application Notes |
|---|---|---|---|
| 200HP-225HP | Mercury / Mariner 3.0L V6 | EFI | Select 3.0L thermostat cover applications |
| 200HP-300HP | Mercury / Mariner 3.0L V6 | DFI / OptiMax | Select 3.0L DFI and OptiMax cooling systems |
| 225HP-300HP | Mercury Pro XS | 3.0L High Performance V6 | Select Pro XS thermostat cover applications |
This thermostat cover kit is commonly serviced together with the thermostat during cooling system maintenance.
View Pro Marine 130°F Thermostat
We also stock the alternate thermostat version:
View Pro Marine 130°F Thermostat 9004P
OEM Part Numbers & Cross References
| Type | Part Number |
|---|---|
| Mercury OEM Number | 885599A1 |
| Mercury OEM Cross Reference | 858856 |
| Pro Marine Item Number | 9004K |
What This Thermostat Cover Kit is Used For
This Mercury thermostat cover kit is used to secure and seal the thermostat assembly inside compatible Mercury 3.0L V6 outboards.
Proper thermostat cover sealing helps maintain correct cooling system pressure, water flow, and engine operating temperature.
Corrosion, damaged sealing surfaces, stripped hardware, or leaking gaskets can contribute to cooling system leaks and temperature regulation problems.
Thermostat cover kits are commonly replaced during thermostat service, cooling system rebuilds, or overheating diagnosis.
Key Features and Benefits
- OEM Mercury thermostat cover kit
- Fits select Mercury and Mariner 3.0L EFI, DFI, and OptiMax outboards
- Compatible with select Mercury Pro XS 3.0L applications
- Replaces Mercury OEM numbers 885599A1 and 858856
- Helps maintain proper thermostat sealing
- Supports proper cooling system operation
- Excellent replacement for corroded or leaking thermostat covers
- Designed for marine outboard cooling systems
Thermostat Cover Kit Details
- Brand: Mercury Marine
- Part Number: 9004K
- OEM References: 885599A1, 858856
- Applications: Mercury & Mariner 3.0L V6 Outboards
- Compatible Systems: EFI, DFI, OptiMax, Pro XS
- Engine Family: 3.0L V6
- System: Cooling System
- Use: Thermostat sealing and cover service
FAQ
What engines does this thermostat cover kit fit?
This kit fits select Mercury and Mariner 3.0L EFI, DFI, OptiMax, and Pro XS V6 outboards using the listed OEM references.
Is this an OEM Mercury part?
Yes. This is an OEM Mercury thermostat cover kit.
What OEM numbers does this replace?
This thermostat cover kit replaces Mercury OEM numbers 885599A1 and 858856.
Should the thermostat also be replaced during service?
In many cases yes. Thermostat covers and thermostats are commonly serviced together during cooling system maintenance.
What happens if the thermostat cover leaks?
A leaking or damaged thermostat cover can contribute to cooling system pressure loss, water leaks, and improper engine temperature regulation.
Dealer Discounts & Store Policies
Dealer / Marine Industry Discounts
Are you a marine dealer, technician, boat builder, or industry professional? We offer dealer pricing for qualified marine businesses.
Shipping Policy
- Free shipping on orders over $100 continental U.S.
- Orders ship via UPS, USPS, and FedEx.
- Orders received before 5:30 PM EST ship the same business day.
- Typical delivery time is 1–5 business days.
30-Day Return Policy
- Returns accepted within 30 days of delivery.
- Cooling system parts should be installed only after proper diagnosis and inspection.
- Items must be new, unused, and in original packaging.
- Return authorization is required before sending items back.
Need help with a return or order question?
Contact Us
Phone: 941-721-3000
Searchable Terms
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885599A1
96233A2
96455A15
96455A15
96455A18
96455T18
96220A2
96220A4
96220A7
96220A13
892864T04
8M0174561
892864T04
8M0233140
880609