Yamaha - Lower Unit Assembly Outboard Drivetrain 6N0-45300-10-4D. Overview. If you've ever dealt with a worn-out lower unit, you know how quickly performance drops off-poor shifting, vibration, and lost efficiency. This Yamaha lower unit assembly is built to bring everything back to smooth, dependable operation. It's designed specifically for smaller Yamaha outboards, so fitment is right and installation stays straightforward. Whether you're running calm inland waters or pushing through rougher