Mercury - Arm-Interlock - 8199321. Ensure precise, secure engine operation with the Mercury - Arm-Interlock - 8199321. This genuine OEM replacement part is engineered for seamless integration into Mercury outboard systems, helping maintain proper shifting function and operational safety. It's a small component with a big role-keeping your marine controls responsive and reliable.. Key Features. OEM Precision Fit - Designed by Mercury to meet exact specifications for dependable performance. Durabl