Yamaha - Gear Case Assembly Lower Unit Component YA2-63705-00-08. Overview. When a lower unit starts making noise or you're dealing with gear wear, it's usually more than a quick fix. I've worked on plenty of Yamahas where replacing the full gear case was the cleanest way to get everything back to factory performance. The Yamaha Gear Case Assembly YA2-63705-00-08 brings the drivetrain back to solid, dependable operation. It's the kind of part you rely on when you want smooth power transfer and c