Yamaha - Lower Unit Assembly F90/F100 Lower Unit 6D9-45300-14-8D. Overview. If you've spent any time running a Yamaha on the water, you know the lower unit is where everything comes together. This assembly is built to handle the everyday stress of shifting, throttle response, and long runs without hesitation. It's the kind of component you install when you want confidence every time you drop it into gear. Whether you're cruising, working, or fishing, it keeps things running smooth and predictabl