Yamaha - Lower Unit Assembly Gearcase Component 6P3-45300-21-8D. Overview. On these larger Yamaha motors, the lower unit is where all that power finally hits the water. I've worked on enough of them to know that when something feels off-noise, vibration, or slip-it usually traces back here. Swapping in a complete assembly like this takes the guesswork out and gets everything back to factory feel. It's the kind of fix that brings your confidence back the moment you throttle up.. Key Features. • Complete Gearcase Replacement - Provides a full lower unit assembly with matched internal components.• Consistent Power Delivery - Transfers engine output smoothly to the propeller for reliable performance.• OEM Engineered Fit - Built to Yamaha specifications for proper alignment and operation.• Heavy-Duty Marine Construction - Designed to withstand high loads and harsh marine environments.. Engine & Boat Compatibility. Designed for Yamaha outboard engines, including high-horsepower LF series mod