When you service a Mercury gearcase, the bearing carrier is one of the most important assemblies inside the lower unit—and on many 4-inch small diameter gearcases, it’s retained by a large threaded cover nut / retainer nut. Removing that nut without the correct tool often leads to chewed-up notches, damaged threads, broken tools, and wasted time.
 
The Buckshot Racing #77 Cover Nut Tool is a purpose-built solution designed to remove the bearing carrier retainer nut (cover) on Mercury’s smaller 4" diameter gearcases, while saving the carrier, the housing, and your knuckles. It’s a direct replacement for Mercury OEM tool/part number 73688, and it delivers the same job-specific function at a fraction of the cost—$99 retail versus the typical $316.99 MSRP for the OEM tool.
 
What this cover nut tool does
This tool is designed to engage the bearing carrier retainer nut (cover) so you can safely apply torque to break it loose during disassembly and reinstall it correctly during assembly. On small-case Mercury lower units, this nut is often tight, corrosion-stuck, or has been over-torqued over the years. A proper cover nut tool distributes load evenly and keeps the tool locked into the nut, which helps prevent slipping and prevents damage to the notches and threads.
 
Why the right tool matters on XR6 small diameter gearcases
The XR6 case and other small-diameter XR6 lower units are common on performance and tournament boats because they are lightweight, efficient, and widely available. They are also frequently used today in outboard drag racing and bass fishing tournaments, where quick turnaround service and reliable gearcase builds matter.
 
Using improvised tools (punches, chisels, pipe wrenches) can scar the retainer nut, distort the carrier, and damage the gearcase—turning a normal bearing carrier service into a full parts-replacement situation. A dedicated Mercury cover nut tool is the difference between a clean rebuild and a damaged housing.
 
Fits Mercury 4" small diameter gearcase families
This cover nut tool is intended for Mercury’s smaller 4-inch diameter gearcases, including the gearcase families most often referenced in service and performance circles. It fits Mercury Inline Six applications using the small-case gearcase designs used in APBA Vintage Outboard Racing, and it fits popular V6 small-case platforms, including XR4 and the XR6 case, yes including smaller diameter XR6 lower units used in outboard drag racing.
 
If your gearcase uses the bearing carrier retained by a threaded cover/retainer nut in a 4" class housing, this tool is the correct style for removal and installation.
 
OEM tool replacement and cost comparison
This tool is built as a replacement for Mercury OEM part number 73688, which is commonly listed around $316.99 MSRP. The Buckshot Racing #77 alternative is priced at $99, making it a practical option for home builders, race teams, and shops that need the tool on hand without tying up budget in high-cost OEM tooling.
 
Basic usage tips (clean, safe results)
For best results, clean the exposed threads and nut area before applying torque. If the carrier has corrosion, penetrating oil and controlled heat (where appropriate) can help. Seat the tool fully into the retainer nut so the engagement is complete, then apply steady torque rather than impact-style hits that can slip and damage the nut. During reassembly, always follow the correct service procedure for your exact gearcase build.
 
The Buckshot Racing #77 Cover Nut Tool is the smart way to remove and install the bearing carrier retainer nut (cover) on Mercury 4" small diameter gearcases, including the Inline Six, Tower of Power, 1500XS, XR4, and XR6 case families.
 
It’s built as a cost-effective replacement for Mercury OEM 73688, helping you service small-diameter XR6 lower units cleanly—whether you’re building for outboard drag racing or maintaining a gearcase for bass fishing tournament reliability.
 
Replaces Mercury OEM Part Number 73688 (MSRP $316.99) • Buckshot Racing #77 Tool $99
 
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.