Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6EK-45300-03-8D. Overview. You can tell right away when a lower unit isn't right-noise, vibration, or just not getting the push you're used to. I've replaced enough of these to know a complete assembly saves a lot of time and guesswork compared to rebuilding one piece at a time. This is the part that takes all the load under the water, so it needs to be solid every time you throttle up. Once it's bolted on and filled, everything just feel