The Buckshot Racing #77 Fat Shaft Gearcase Oil Seal is a premium-quality replacement for high-performance Mercury Racing applications using the Mercury OEM 815565 seal. Designed for extreme marine duty, it delivers precise sealing for 1.25-inch (1-1/4") “fat shaft” propeller systems used in Mercury Sportmaster, Torquemaster, and Bravo XR/XZ lower units.
As a direct-fit replacement for Mercury OEM 26-815565, this seal is built to exact factory specifications. Its precise tolerances ensure proper interference fit, lip geometry, and load-handling capability, making it the ideal choice for both original equipment service and high-performance upgrades.
Outboard Applications
The Buckshot Racing #77 seal is engineered for Mercury Racing outboards equipped with fat shaft Sportmaster or Torquemaster lower units. Compatible models include the 200 HP XS 2.5-liter six-cylinder, the 225 HP XS and 225 HP Pro XS three-liter engines, the 225 HP Sport XS 2.5-liter, and the 225X EFI ProMax and 225XB EFI three-liter versions.
It also fits the 250 HP XS and 250 HP Pro XS three-liter configurations, the 250 HP Sport XS 2.5-liter, and the 250X EFI ProMax and 250XB EFI three-liter models. For larger displacement applications, it is a precise fit for the 250 HP Pro XS 3.2-liter, the 300 HP XS 3.2-liter, the 300X EFI Pro Max 3.0-liter, and the 300 EFI SuperMagnum 3.0-liter. It is also used on select 150, 200, and 225 HP Pro Max and Super Mag EFI engines.
Outdrive & Sterndrive Applications
Beyond outboards, this seal is a proven fit for Mercury Racing sterndrive platforms utilizing Bravo XR and XZ configurations. It supports Bravo XR Sportmaster, Bravo XR One, Bravo XR Three, and Bravo XR Sport lower units, as well as Hi-Performance Integrated Transom assemblies.
These applications place the seal under substantial thrust loads and side forces generated by large-diameter propellers and high-output powerplants, demanding absolute sealing integrity during aggressive acceleration, sustained cruising, and offshore endurance runs.
Material & Construction
Constructed from high-grade Nitrile Rubber (NBR), the Buckshot Racing #77 seal provides exceptional wear resistance and compatibility with petroleum-based hydraulic fluids, high-performance gear oils, alkalis, acids, and hydrocarbon cleaning solvents. The internal spring-loaded lip maintains consistent shaft contact across the entire RPM range, ensuring leak-free operation from idle through maximum engine output.
Non-Magnetic Design
One of the defining engineering advantages of the Buckshot Racing #77 Fat Shaft Gearcase Oil Seal is its fully non-magnetic construction. Unlike conventional magnetic seals, which can attract ferrous particles suspended in the water or oil, this design eliminates particle accumulation at the sealing surface.
In saltwater and brackish environments, where electrolytic activity is a constant threat, the absence of magnetic attraction significantly reduces galvanic corrosion within the gearcase assembly. This not only extends the service life of the seal itself but also protects the propeller shaft, bearing carrier, and associated drivetrain components from premature wear and structural degradation. The result is a sealing system optimized for long-term durability in the harshest marine operating conditions.
Application Summary
The Buckshot Racing #77 Fat Shaft Gearcase Oil Seal—Mercury OEM 815565—is purpose-built for Mercury Racing outboards and Bravo-series sterndrives requiring fat shaft sealing solutions. From lightweight 2.5-liter drag setups to large-displacement offshore powerplants, it delivers uncompromising performance, extended service life, and superior corrosion resistance for serious racers and performance boaters.
Includes one (1) Propshaft Seal for the Mercury 1.25-inch Fat Shaft.
- Select 2 Seals, Save $4
- Select 10 Seals Dealer Pack, Save $40
Compared to Mercury OEM Part Numbers: 26-815565, 815565
Contact Mike Hill at mike@buckshotracing77.com w/ technical questions