The Lower Unit Thread Chaser Tools from Buckshot Racing #77 offer a professional solution for restoring damaged bearing carrier retainer nut threads in Mercury outboard gearcases. Saltwater corrosion, cross-threading, or mechanical wear can compromise the internal threads, but this kit restores them to factory specifications—eliminating the need for full lower unit replacement.
 
This updated kit includes three precision-engineered chasers, each tailored to a specific Mercury gearcase diameter: 4.000″ (ID), 4.375″ (ID), and 5.44″ (OD). Machined from high-strength tool steel, each follows OEM thread specs and resists corrosion, ensuring long-lasting durability even in saltwater environments.
 
4.000″ Thread Chaser – Mercury XR4 & XR6 Small Gearcases
 
Designed for Mercury’s smaller 4.000″ inner-diameter gear housings, this chaser is ideal for early XR4 and XR6 lower units found on mid-range V6 outboards from the 1980s to the 1990s. Machined to 4.000″‑16 UNC, the tool restores compact OEM thread profiles, addressing issues caused by cross-threading, over-torquing, or corrosion.
 
This chaser preserves the hard-to-find smaller bullet gearcases commonly used for outboard drag racing by reestablishing factory-specified threads for secure retainer nut installation.
 
4.375″ Thread Chaser – Standard 150-200HP, CLE, and Sportmaster Gearcases
 
Targeting Mercury’s most common 4.375″ (4 3/8″) gear housings, this chaser suits a wide range of V6 outboards (135–300 HP), including the standard production cases and high-performance CLE and Sportmaster units.
 
Compatible models include Black Max, XR2, XR4 (standard), larger XR6, XRi, Pro Max (150–225 HP), 260 EFI, 280 ROS, and 300 Drag, as well as both EFI and carbureted 200–250 HP V6 engines. Commercial and sterndrive platforms like SeaPro, Marathon, DTS 6‑cyl, Alpha One, R/MR Alpha, and Bravo XR are also supported.
 
The tool, machined to 4.375″‑16 UNC, re-cuts threads damaged by galling, misalignment, or corrosion. It aligns with Mercury part numbers 79448, 8M0168577, 8M2000347, 8M2011027, and performance variants CLE 15840 and Sportmaster 158401.
 
5.44″ Thread Chaser – Verado & Mercury Racing Offshore Gearcases
 
This heavy-duty chaser is designed for the wide 5.44″ torpedo-style gearcase, utilized across both Mercury Verado (V6 and V8, 200–350 HP) and Mercury Racing outboards like the 300R, 400R, and 450R. Although the engines differ in performance, they share this robust gearcase platform.
 
The 5.44″ gearcase enhances hydrodynamic lift, propeller clearance, and cooling—ideal for offshore center consoles, high-speed catamarans, and tournament boats. It supports both Sportmaster and 5.44 HD versions, available in 20″–30″ shaft lengths, gear ratios 1.60, 1.75, and 1.85, and standard or counter-rotation configurations.
 
This chaser allows rebuilders to restore the bearing carrier retainer threads in these demanding gearcases—saving costly replacements while maintaining structural integrity.
 
How to Use the Buckshot Racing #77 Thread Chasers for Mercury Gearcase Repair
 
Begin by thoroughly cleaning the damaged threads using a wire brush and degreaser. Apply a true cutting oil—such as Tap Magic or Ridgid—to reduce friction, protect the aluminum housing from galling, and improve cutting performance.
 
Carefully align the appropriate chaser perpendicular to the housing face. Using a bearing carrier nut wrench (Buckshot or equivalent), turn slowly clockwise, allowing the cutting teeth to engage smoothly.
 
Avoid forcing the tool. Periodically back it out to remove metal chips and reapply cutting oil. Once the thread is fully restored, clean the housing and inspect for clean, uniform threads ready to accept the bearing carrier nut.
 
Professional-Grade Tools for Mercury Lower Unit Rebuilds
 
The Buckshot Racing #77 Thread Chaser Tool combines precision, durability, and OEM compliance—making it a must-have for marine rebuilders,
 
Restore confidence—and your threads—with the only toolset engineered for precision Mercury outboard gearcase thread repair.
 
Contact Mike Hill with any questions at +1-714-697-1716 or mike@buckshotracing77.com