Yamaha - Lower Unit Assembly Outboard Drivetrain 6M8-G5300-10-4D. Overview. You can usually tell when a lower unit is on its way out-things get noisy, shifting feels off, and the motor just doesn't run as clean. Swapping in a fresh Yamaha assembly like this brings everything back to how it should feel on the water. It bolts up right, runs smooth, and gives you that solid, predictable performance you expect. It's the kind of part you install once and trust every time you head out.. Key Features.