The Buckshot Racing #77 Cylinder Bore O-Ring & Head Seal Kit is designed to replace Mercury OEM Part Numbers 884391 (Bore Seals) and 818574 (Outer Seal) whenever a cylinder head is removed from a Mercury 3.0L V6 outboard.
Each kit services one (1) cylinder head and includes three (3) replacement cylinder bore O-rings and one (1) replacement outer cylinder head seal. Since every Mercury 3.0L V6 engine uses two cylinder heads, two kits are required when resealing an entire engine.
Fits Mercury outboards with serial number 0T651945 and higher.
Replacing these seals during every cylinder head service helps maintain proper combustion sealing, coolant containment, and long-term engine reliability.
Direct Replacement for Mercury OEM Parts
This Buckshot Racing #77 kit replaces the factory Mercury sealing components used on a wide range of Mercury 3.0L engines. The three replacement cylinder bore seals take the place of Mercury OEM Part Number 884391, while the perimeter head seal replaces Mercury OEM Part Number 818574.
Every seal is manufactured to meet or exceed original specifications, providing the proper fit and sealing performance expected during engine rebuilds, routine maintenance, or cylinder head repairs.
Mercury's 3.0L engines do not use a traditional composite head gasket. Instead, combustion pressure is sealed by individual cylinder O-rings while a separate outer seal contains the cooling system around the cylinder head. Installing new seals whenever the head is removed is considered standard service practice and helps prevent coolant leaks, compression loss, and overheating.
Fits Numerous Mercury 3.0L & 3.2L V6 Engines
The Buckshot Racing #77 replacement kit fits an extensive range of Mercury Marine and Mercury Racing 3.0L & 3.2L V6 outboards, including carbureted, EFI, DFI, Optimax, and commercial models.
Common applications include Mercury 200 HP, 225 HP, 250 HP, and 300 HP outboards equipped with the 3.0L V6 powerhead. It is also compatible with many 225 EFI, 250 EFI, 225 Optimax, 250 Optimax, 300 Optimax, 225 Pro XS, 250 Pro XS, 300 Pro XS, 225 Sport XS, 250 Sport XS, 250XS, 300X, 300XS, SeaPro, Marathon, Commercial, SportJet 250 EFI, SportJet 250 DFI, and SportJet DFI M2 applications.
Compatibility also extends to numerous Mercury DTS, Verado-era 3.0L commercial engines, and other production variations listed by Mercury. Because Mercury produced many serial-number changes throughout the years, engine serial number verification is always recommended before ordering.
Why Replace the Seals?
Cylinder head seals are compressed every time the heads are torqued into place. Once removed, the original seals should not be reused. Installing new replacement seals helps maintain proper cylinder compression while preventing combustion gases from entering the cooling system or coolant from leaking externally.
Many overheating complaints and unexplained cooling system pressure problems can often be traced to damaged cylinder O-rings or worn perimeter seals after cylinder head service. Replacing these inexpensive sealing components during reassembly greatly reduces the likelihood of future failures.
Perfect for Engine Rebuilds and Performance Maintenance
The Buckshot Racing #77 Cylinder Head Seal Kit is recommended whenever performing piston replacement, sleeve installation, cylinder head resurfacing, powerhead rebuilding, ring replacement, cooling system repairs, compression inspections, or high-performance engine modifications.
Whether you're rebuilding a Mercury 300XS, refreshing a 250 Pro XS, servicing a 225 Optimax, or maintaining a commercial SeaPro engine, replacing the cylinder bore O-rings and outer head seal should always be part of the repair.
Installation Recommendations
Before installing the new seals, thoroughly clean the cylinder head and block sealing surfaces. Carefully inspect each O-ring groove for corrosion, nicks, or debris that could affect sealing. Lightly lubricate the new cylinder bore O-rings with clean two-stroke oil before installation and torque the cylinder head using the sequence and specifications provided in the appropriate Mercury service manual.
Always inspect the cylinder head for flatness before reassembly, particularly on engines that have experienced overheating.
Buckshot Racing #77 Tech Tip
Unlike traditional head gaskets, Mercury's 3.0L V6 design relies on precision cylinder bore O-rings and a separate outer water jacket seal. Reusing these seals after cylinder head removal can increase the risk of coolant leaks, combustion leakage, and uneven cylinder sealing.
Because each kit services one cylinder head, remember that two Buckshot Racing #77 kits are required to completely reseal both heads on every Mercury 3.0L V6 engine.
Final Thoughts
The Buckshot Racing #77 Mercury 3.0L Cylinder Bore O-Ring & Head Seal Kit provides a dependable replacement for Mercury OEM 884391 and 818574, giving engine builders and technicians the sealing components required whenever a cylinder head is removed.
Designed for numerous Mercury 200 HP, 225 HP, 250 HP, and 300 HP 3.0L EFI, DFI Optimax, Pro XS, Sport XS, 300X, 300XS, SeaPro, Marathon, SportJet, and commercial V6 outboards, this kit delivers the quality, fit, and reliability needed for both stock and high-performance applications.
For the best results, always replace the seals rather than reusing compressed originals, and verify engine serial number compatibility before installation.