Yamaha - Lower Unit Assembly Gearcase Component 6N0-45300-20-4D. Overview. On any outboard, the lower unit is where the real work happens once power leaves the engine. I've seen boats lose smoothness and efficiency just from wear down there, even when everything else checked out fine. When you install a solid assembly like this, you feel it right away-clean engagement, quieter operation, and better push through the water. It's the kind of upgrade that brings confidence back every time you drop it into gear.. Key Features. • Complete Gearcase Assembly - Provides a full replacement with properly matched internal components.• Efficient Power Transfer - Delivers smooth and consistent output to the propeller.• OEM Precision Fit - Built to Yamaha specifications for correct alignment and installation.• Corrosion-Resistant Construction - Designed to handle harsh freshwater and saltwater environments.. Engine & Boat Compatibility. Designed for Yamaha outboard engines requiring dependable gearca