Yamaha - Gear 1 Assembly Lower Unit Drivetrain 6E5-45560-01-00. Overview. If you've ever torn into a Yamaha lower unit, you know the gears are where everything either comes together-or falls apart. This Gear 1 Assembly plays a key role in transferring power smoothly from the driveshaft to the prop, especially when you're shifting under load or cruising for long stretches. It's the kind of part you don't think about until shifting gets rough or you start hearing something you shouldn't. Built for