Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6EM-45300-00-8D. Overview. When a lower unit starts going, you feel it right away-noise, vibration, or just not getting the push you're used to. I've swapped out enough of these to know that a solid, complete assembly saves time and headaches compared to piecing things together. This is the part of the motor that takes all the abuse below the waterline, so it needs to be right. Bolt it on, fill it up, and you're back to running smooth and