The Buckshot Racing #77 Mercury V6 End Cap O-Ring Kit replaces the two critical sealing O-rings used in the crankshaft carrier assemblies on legacy Mercury 2-stroke V6 outboards. This kit includes the Lower End Cap O-Ring Mercury part numbers 81520, 508634, and 62700, along with the Upper End Cap O-Ring Mercury part numbers 508636 and 54029.
 
These seals are used throughout the classic Mercury 2.0-liter, 2.4-liter, and 2.5-liter V6 engine platforms, helping maintain proper crankcase pressure and internal sealing around the crankshaft end caps. Engines using these components include many of Mercury’s most recognizable fishing and performance motors built from the late 1970s through the early 2000s.
 
Why the Mercury End Cap O-Rings Matter
The crankshaft carrier caps seal the upper and lower ends of the rotating assembly inside the block. The Lower End Cap O-Ring (Mercury 81520, 508634, 62700) seals the lower carrier where it meets the engine block, while the Upper End Cap O-Ring (Mercury 508636, 54029) seals the top carrier to maintain proper crankcase compression.
 
On a Mercury V6 two-stroke, crankcase pressure is essential for moving the fuel-air mixture through the transfer ports. If the 81520 / 508634 / 62700 lower O-ring or the 508636 / 54029 upper O-ring becomes hardened or flattened, the engine can develop internal air leaks. This can result in unstable idle, lean conditions, poor throttle response, or inconsistent fuel delivery.
 
Replacing the Mercury end cap O-rings 81520, 508634, 62700, 508636, and 54029 during service ensures the crankcase remains properly sealed and the engine operates as designed.
 
Mercury Engines That Use O-Rings 81520 and 508636
The Mercury end cap O-rings 81520, 508634, 62700, 508636, and 54029 are used across a wide range of Mercury 2-stroke V6 engines, especially the 2.0-liter, 2.4-liter, and 2.5-liter platforms.
 
These engines include many well-known models such as Mercury 135 HP, 150 HP, 175 HP, and 200 HP V6 outboards, along with the classic Black Max series. The same sealing components are also found in popular performance and bass-boat engines such as the XR2, XR4, XR6, and XRI models, which remain widely used today.
 
In addition, these O-rings appear in numerous Mercury Racing powerheads, including 2.4 Bridgeport engines, 245 Carb, 260 EFI, 280 ROS, and 300 Drag engines, as well as race motors used in SST-120 and S3000 competition platforms.
 
Because Mercury maintained the same basic crankcase architecture across multiple generations of V6 engines, the Lower End Cap O-Ring 81520 / 508634 / 62700 and the Upper End Cap O-Ring 508636 / 54029 remained common service parts throughout decades of production.
 
Recommended During Engine Service or Rebuild
Whenever the crankshaft carrier caps are removed, replacing the O-rings is recommended. Over time, seals such as 81520, 508634, 62700, 508636, and 54029 can flatten and lose elasticity from heat cycles and compression.
 
During engine rebuilds, installing new Mercury end cap O-rings 81520 / 508634 / 62700 and 508636 / 54029 helps restore proper sealing and prevents crankcase pressure leaks that could affect engine performance.
 
This step is especially important when performing crankshaft replacement, bearing service, front-half removal, or full powerhead rebuilds on Mercury V6 engines.
 
Supporting Classic Mercury V6 Two-Stroke Engines
The legendary Mercury 2.0L, 2.4L, and 2.5L V6 two-stroke engines remain widely used in bass boats, offshore performance hulls, and racing applications. Proper crankcase sealing is essential for these engines to maintain the power and reliability that made them famous.
 
The Buckshot Racing #77 Mercury End Cap O-Ring Kit, containing the Lower End Cap O-Ring 81520 / 508634 / 62700 and the Upper End Cap O-Ring 508636 / 54029, provides the correct replacement seals to keep these classic Mercury V6 engines—including XR2, XR4, XR6, and XRI models—properly sealed and running strong.
 
Kit Includes 1 x Upper End Cap O-Ring (Mercury 508636) and 1 x Lower End Cap O-Ring (Mercury 81520)
 
Contact Mike at 714-697-1716 or mike@buckshotracing77.com for technical support.