he Buckshot Racing #77 O-Ring Kit is a premium replacement for the Mercury Power Trim Cylinder Repair Kit 79879A1. This rebuild kit is designed for servicing single-ram Mercury power trim and tilt cylinders used on many Mercury V6 outboard engines.
 
The kit restores proper hydraulic sealing in the Mercury trim cylinder, eliminating common problems such as trim fluid leaks, slow trim response, and loss of hydraulic pressure. Instead of replacing the entire trim assembly, the Buckshot Racing #77 kit allows boat owners and marine technicians to rebuild the internal seals inside the Mercury trim ram cylinder.
 
Over time, the internal O-rings and seals in Mercury trim cylinders wear out due to heat, pressure, and normal hydraulic cycling. Replacing these seals with the Buckshot Racing #77 kit restores reliable trim and tilt performance while extending the life of the original Mercury trim cylinder.
 
Compatible Mercury Outboard Engines
The Buckshot Racing #77 Mercury trim cylinder rebuild kit, a direct replacement for Mercury kit 79879A1, fits many Mercury and Mariner outboards equipped with a single long trim ram cylinder.
This trim rebuild kit is commonly used on Mercury V6 performance outboards, including the Mercury XR2, XR4, Black Max, Bridgeport, 225 Pro Max, 300X, and 2.5-liter Offshore ROS (Race Offshore Series) engines.
 
The kit supports Mercury V6 two-stroke outboards in 2.0L, 2.4L, 2.5L, and 3.0L displacements, covering engines produced from the late 1970s through the early 2000s.
 
Additional Mercury Models Using This Trim Cylinder
Mercury parts catalog listings show this trim cylinder design used on a wide range of EFI and high-performance Mercury outboards.
 
These include engines such as the 200 EFI Pro Max / Super Mag, 225 EFI Pro Max / Super Mag, 150 Pro Max, 200 Pro Max, and 225X EFI Pro Max models. The rebuild kit also applies to 300 Pro Max (3.0L EFI) and 300X Pro Max (3.0L EFI) engines commonly used in offshore performance boating.
Several Mercury XS performance series outboards also use this trim system, including the 200 HP XS 2.5L V6, 250 HP XS 3.0L V6, and 300 HP XS 3.2L V6 engines.
 
Earlier Mercury V-series outboards equipped with this trim cylinder include the V-150, V-150 Magnum, V-150 Marathon, V-150 XR-2, V-150 Work models, V-1500, V-175, and V-200 engines.
 
Mercury Racing catalogs also reference engines such as the 200 ROS HP 3.4L V6 15-inch midsection racing engine, 300 ROS HP 4.6L V8 15-inch midsection racing engine, 200R HP 3.4L V6 racing engine, and 300R HP 4.6L V8 racing engine that utilize similar trim cylinder components.
 
Mercury Power Trim Cylinder Rebuild Overview
Before rebuilding the trim cylinder, prepare a clean workspace to prevent contamination inside the hydraulic system.
 
Disconnect the hydraulic hoses and remove the trim cylinder following procedures in the Mercury Marine Service Manual. Drain the hydraulic fluid by directing the trim cylinder ports into a container while cycling the trim rod.
 
Using a pin spanner wrench, remove the rod guide and withdraw the trim rod assembly. Remove the floating piston and discard the old O-ring.
 
Next, remove the shock piston assembly from the trim rod. The worn O-rings, springs, spring guides, spring seats, and check balls should all be removed and replaced with new components from the Buckshot Racing #77 trim rebuild kit.
 
Disassemble the rod guide by removing the retaining ring, washer, scraper, and the three O-rings. These seals are replaced during the rebuild process.
 
Mercury Trim Cylinder Reassembly
Clean all reusable components with a non-toxic solvent and dry them using compressed air. Inspect the cylinder bore and trim rod for scoring or wear before reassembly.
 
Lubricate all seals and components with SAE 10W-30 or 10W-40 motor oil during installation.
 
Rebuild the rod guide using the new O-rings, scraper, washer, and retaining ring, then slide the rod guide back onto the trim rod.
 
Reassemble the shock piston using the new check balls, spring seats, compression springs, spring guides, and O-rings. Apply Loctite 271 to the piston bolt and torque it to 58–72 ft-lbs.
 
Install the new floating piston O-ring and reassemble the trim cylinder. After reinstalling the cylinder on the engine, reconnect the hydraulic hoses and cycle the trim system several times to verify smooth trim operation and leak-free performance.
 
Why Rebuild a Mercury Trim Cylinder
The Mercury power trim cylinder controls engine trim angle and plays a critical role in boat performance, fuel efficiency, and handling. When internal seals fail, the trim system may leak hydraulic fluid or operate slowly.
 
Rebuilding the cylinder with the Buckshot Racing #77 kit — a replacement for Mercury trim repair kit 79879A1 — restores hydraulic performance and prevents costly trim system replacement.
 
This rebuild procedure applies to many Mercury and Mariner outboards manufactured from the late 1970s through the early 2000s, making it an effective maintenance solution for keeping classic Mercury outboards operating reliably.
 
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.