Mercury - Quicksilver Gear Lube Drain & Fill Screw with Seal Lower Unit Component 79953Q04. Overview. Keeping your lower unit sealed tight is one of the easiest - and most important - ways to protect your engine's gears. The Mercury Marine 79953Q04 Quicksilver Drain and Fill Screw with Seal is designed for all MerCruiser sterndrives and Mercury/Mariner outboards (except Verado models). This non-magnetic plug and seal combination ensures your gear lube stays in and water stays out. It's a simple