The Buckshot Racing #77 Shift Shaft Bushing & Seal Kit is an assembled replacement kit that includes the shift shaft bushing, seal, and O-ring. This kit replaces Mercury OEM part number 23-77631A 2 and is used on many Mercury outboard lower units and MerCruiser Alpha sterndrives.
 
This part sits where the shift shaft passes through the gearcase. It supports the shaft, helps keep it aligned, and seals the opening so water stays out and gear oil stays in. When the bushing or seal wears, the lower unit can develop water intrusion, gear oil leaks, sloppy shifting, or internal gearcase damage.
 
What the Shift Shaft Bushing Does
 
The shift shaft bushing guides and supports the shift shaft as it moves between forward, neutral, and reverse. Proper alignment is important because the shift shaft controls gear engagement inside the lower unit. A worn bushing can allow excess shaft movement, which may affect shift feel and accelerate wear around the shift shaft opening.
 
What the Shift Shaft Seal and O-Ring Do
 
The shift shaft seal helps keep water, dirt, and contaminants out of the gearcase while keeping gear lube inside. The O-ring seals the outside of the bushing assembly in the gearcase bore.
 
If either seal fails, water can enter the lower unit or gear oil can leak out.
Because the bushing, seal, and O-ring work together, they should usually be serviced as a complete assembly instead of replacing only one piece.
 
Common Failure Symptoms
 
A worn shift shaft bushing or seal often shows up as milky gear oil, gear lube leaking near the shift shaft, pressure-test failure, vacuum-test failure, or repeated water contamination after fresh lube is installed.
 
You may also notice loose shift shaft movement, corrosion near the shift shaft opening, or shifting that does not feel as clean as it should.
 
Any lower unit with water in the oil should be pressure and vacuum tested before being returned to service.
 
Fitment and Cross-Reference Part Numbers
 
Buckshot Racing #77 offers replacement shift shaft bushing and seal solutions for many Mercury 35 HP to 300 HP outboards, including common gearcase families and applications such as Sport Master, Torque Master, CLE, XR4, XR6, Black Max, 150 HP, 175 HP, 200 HP, XRi and Fleet Master, along with many MerCruiser Alpha sterndrives.
Common replacement and cross-reference numbers include Mercury 23-77631A 2, 18-2155, 21020, 37428A1, 508708, and 55419A1.
 
Always confirm fitment by engine model, serial number, gearcase style and application before ordering. Mercury lower units vary across horsepower ranges, years and gearcase families.
 
Optional Removal and Installation Tool
 
The optional shift shaft bushing removal and installation tool replaces Mercury tool number 91-31107. It is designed to remove and install the shift shaft bushing correctly in Mercury gearcases and MerCruiser Alpha sterndrives.
 
Using the right tool helps keep the bushing square during service and reduces the risk of damaging the gearcase, shift shaft, bushing bore or seal area. Improvised methods can nick the housing, distort the bushing, damage the seal or install the part crooked, which can lead to another leak later.
 
Installation Notes
 
Before installing the new kit, inspect the shift shaft for grooves, pitting, corrosion or wear where the seal rides. A new seal cannot work correctly on a damaged shaft. The gearcase bore should also be clean and free of corrosion, burrs, and debris.
 
After installation, pressure and vacuum test the lower unit, refill it with the correct gear lube, check shifting operation, and inspect for leaks after the first run.
 
Why This Kit Matters
 
The shift shaft bushing and seal kit is a small but critical lower-unit service part. If it fails, water can contaminate the gear oil, lubrication can be lost, and expensive gears, bearings, driveshafts, or propshafts can be damaged.
 
Replacing the bushing, seal, and O-ring during gearcase service is cheap insurance compared to replacing a complete lower unit.
 
Buckshot Racing #77 Tech Tip
 
Any time a Mercury lower unit has milky gear oil, shift shaft play, or a pressure-test leak near the shift shaft, inspect the shift shaft bushing and seal assembly. Replace the bushing, seal and O-ring together, and use the correct removal and installation tool whenever possible.
 
A clean, square installation helps protect the gearcase, maintain proper shifting and keep the lower unit sealed.
 
Message or call us at 714-697-1716 with your lower unit type if you have any questions.