Yamaha - Lower Unit Assembly 25/30 HP Complete Drivetrain Assembly 6J8-45300-14-4D. Overview. The Yamaha Lower Unit Assembly 25/30 HP 6J8-45300-14-4D is what takes everything your engine produces and actually puts it into the water. When a lower unit starts going, you'll feel it fast-noise, vibration, or loss of power at the prop. This is a full assembly built for those 25 and 30 horsepower motors, so you're not piecing things together or guessing on fit. If you're dealing with major lower unit