The Mercury OEM Wrist Pin Clip Lock Ring Installer Tool (part number 91-77109A3) is a factory tool designed to install wrist pin retaining clips—also known as circlips, snap rings, or "Jesus clips"—on Mercury and Mariner 2-stroke engines using small wrist pins.
 
It aligns and inserts the clip in a single operation, reducing installation errors and preventing distortion. Incorrectly installed clips can lead to wrist pin migration and severe engine damage, especially in high-RPM applications like the 2.5L and 3.0L Mercury Racing platforms.
 
These include engines such as the V-150 HP, V-175 HP, V-200 EFI, 135 HP V6, and 150 HP V6 (2.5L). The 91-77109A3 tool ensures the clip is seated squarely in its groove, critical to mechanical integrity in these powerheads. It is used across Mercury’s performance and utility product lines, including SportJet models such as the 120XR2, 175XR2, 200 DFI M2, 240 EFI, and 250 DFI.
 
These engines operate in high-load, vibration-prone environments where clip retention must be precise. Technicians rebuilding legacy motors like the XR6, Magnum III, or V6 outboards from 135 to 200 HP (serial range 0G303046–0G760300) will find this tool equally applicable. It's also used on 4-cylinder models such as the 115 HP and 125 HP, and inline 3-cylinders like the 75 HP, 90 HP, and 50 HP, as well as smaller 2-cylinder engines including the 30 HP and 40 HP.
 
The installer is backward-compatible with earlier part numbers, including 91-77109A1, 77109A3, and 60837A1, supporting multiple generations of Mercury piston and wrist pin designs. Installation Instructions for 91-77109A3:
 
1. Install Wrist Pin (Partial) Insert the wrist pin through the piston and rod, stopping short of the opposite bore. Leave enough room to install the retaining clip.
 
2. Load Clip into Tool Place the clip into the tool's jaws. Align the clip gap with the tool’s slot. Proper orientation is critical—aim for 12 or 6 o’clock in the piston groove.
 
3. Position Tool Insert the tool tip into the piston bore. Hold it square against the piston wall with light pressure.
 
4. Install Clip Depress the tool’s plunger or handle to push the clip into the groove. The tool ensures even expansion and full seating without distortion.
 
5. Inspect Seating Visually confirm that the clip is fully seated in the groove. The wrist pin should have no side-to-side movement.
 
6. Repeat (if needed) Push the wrist pin fully through and repeat the process on the opposite side, if a second clip is required.
 
This procedure helps ensure the correct wrist pin clip installation in Mercury engines from high-output racing motors to production outboards and jet drives. In environments where vibration and load are constant, proper clip retention is essential to avoid internal engine failure. 
 
Contact Mike Hill at mike@buckshotracing77.com or +1-714-697-1716 for tech support