Mercury - Quicksilver Exhaust Flapper Kit Exhaust System Component 4-Inch Through-Transom 12644A1. Overview. Every boater knows that distinctive rumble of a through-transom exhaust - the sound that says your engine's alive and breathing right. The Mercury Marine 12644A1 Quicksilver Exhaust Flapper Kit helps keep that performance sound crisp while protecting your system from unwanted backflow. Designed for 4-inch through-transom outlets, this flapper guards your exhaust path from water intrusion