ATAT Performance proudly offers a precision-engineered Billet Top Cap Plate, CNC machined from aerospace-grade 6061-T6 billet aluminum, specifically designed for Mercury Racing 2.5-liter outboards equipped with the big bearing crankshaft.
 
This high-performance billet plate is a direct replacement for OEM part number 849366T1 and is designed to fit Mercury crankshaft #848330T4, found in engines like the 280 ROS (late models), 300 Drag, and S3000.
 
Built for strength, accuracy, and durability, the ATAT Billet Top Cap provides superior crankshaft support and alignment under high-RPM conditions. The billet construction offers a nice upgrade over the OEM cast cap.
 
This top cap is machined to tighter tolerances than stock, ensuring optimal load distribution and crankcase sealing. The corrosion-resistant anodized finish enhances longevity and performance in harsh marine environments.
 
Mercury transitioned to a larger upper crank bearing in high-output models like the 280 ROS, 300 Drag, and S3000 to handle increased loads and RPM. The OEM 849366T1 top cap was introduced to accommodate the larger bearing found in the 848330T4 crankshaft. A
 
TAT Performance’s billet version retains the OEM geometry but enhances structural integrity through the use of 6061 billet aluminum and advanced CNC machining processes.
 
Whether you're pushing a 2.5-liter Mercury to the limits in drag racing, circuit competition, or high-speed lake runs, this top cap is built to perform. It is a drop-in replacement that provides the strength, reliability, and precision that serious racers demand.
 
If you're currently running or rebuilding a big bearing Mercury 280 ROS, S3000, or 300 Drag, this billet top cap is a critical upgrade that ensures long-term reliability and crankshaft alignment.
 
Designed and manufactured in Australia by ATAT Performance, this billet top cap is a race-bred solution developed in partnership with real-world performance builders.
 
For those looking to replace Mercury OEM 849366T1 and run the 848330T4 big bearing crankshaft, there’s no better choice than ATAT’s billet top cap—precision, performance, and peace of mind, all in one part.
 
The initial run of replacement 849366 top plates are offered at the RacerNet special of $159!
 
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.