One of the most common problems encountered when servicing a Mercury Racing 2.5L ROS (Race Offshore) outboard is discovering that standard fishing motor lower unit studs are too short.
The reason is simple—Mercury Racing ROS midsections use a thicker exhaust can, requiring longer studs to achieve proper thread engagement.
The Buckshot Racing #77 Lower Unit Stud Kit is specifically engineered for 15-inch and 20-inch Mercury 2.5L ROS midsections equipped with the thicker racing exhaust can. Unlike standard production studs, these longer fasteners provide the proper clamping force needed to securely retain the lower unit under extreme horsepower and high-RPM operation.
Built to Mercury Racing Specifications
At the heart of the kit are five OEM-spec 7/16-14 lower unit studs that replace Mercury OEM 16-59579, now superseded by Mercury 8M2016987.
Each stud measures approximately 2.812 inches (2-13/16") in overall length, providing the additional thread engagement required for Mercury Racing applications using thicker adapter plates and exhaust cans.
The kit also includes five hardened OEM-spec case-hardened washers and five new stainless steel prevailing-torque lock nuts, allowing you to replace all critical mounting hardware during a lower unit installation or rebuild.
Why Longer Studs Matter
The lower unit is subjected to tremendous loads during operation. Every launch, hard acceleration, gear shift, propeller impact, and high-speed pass places stress on the fasteners that secure the gearcase to the midsection.
Studs that are too short may not fully engage the lock nuts, reducing clamping force and increasing the likelihood of the fasteners loosening over time.
Proper thread engagement is essential for maintaining even clamping pressure across the gearcase and adapter plate.
Using the correct-length studs helps maintain gearcase alignment while reducing the risk of movement under heavy propeller loads.
Prevent Expensive Lower Unit Damage
Loose or improperly installed lower unit hardware can lead to much more than an oil leak. As clamping force decreases, the gearcase can begin to shift during operation, placing additional stress on the driveshaft, shift shaft, water tube, adapter plate, and midsection.
In severe cases, movement between the lower unit and midsection can contribute to damaged mating surfaces, broken cavitation plates, fractured midsections, damaged driveshaft splines, water pump failures, and even complete gearcase loss.
Replacing worn, corroded, stretched, or incorrect studs is inexpensive insurance whenever the lower unit is removed.
Ideal for Mercury Racing Gearcases
The Buckshot Racing #77 stud kit is designed for Mercury Racing applications equipped with SportMaster and CLE (Crescent Leading Edge) gearcases, including many 2.5L ROS, 260 EFI, 280 ROS, Drag, S3000, Formula Race, and modified high-performance Mercury V6 engines using the thicker ROS exhaust can.
Because Mercury produced several midsection and adapter plate combinations, always verify your application before ordering.
Buckshot Racing #77 Tech Tip
Whenever installing lower unit studs, thoroughly clean the threaded holes in the adapter plate and inspect for damaged threads or corrosion.
Install the studs to the proper depth using the Mercury service manual, replace the lock nuts rather than reusing old hardware, and torque all fasteners evenly using the factory tightening sequence. Uneven clamping can lead to gearcase misalignment and premature component wear.
Kit Includes
- Five (5) OEM-spec 7/16-14 × 2.812-inch lower unit studs
- Five (5) hardened case-hardened flat washers
- Five (5) stainless steel prevailing-torque lock nuts
Final Thoughts
The Buckshot Racing #77 Mercury 2.5 ROS Lower Unit Stud Kit provides the correct hardware for Mercury Racing midsections that require longer mounting studs.
Replacing Mercury OEM 16-59579 and 8M2016987, this complete kit restores proper thread engagement, maintains secure lower-unit clamping, and helps protect valuable SportMaster and CLE gearcases from movement under extreme operating conditions.
Whether you're rebuilding a 2.5L ROS, servicing a SportMaster, restoring a 2.4L CLE, or maintaining a high-performance Mercury Racing outboard, installing the correct lower unit studs is a simple upgrade that improves both reliability and long-term durability.
Contact Mike Hill at +1-714-697-1716 for technical support.