Mercury 12-191833 - Drain Plug Screw Seal Gasket. OEM Gearcase Drain/Fill Screw Seal Factory Composite Washer For Mercury, Mariner & MerCruiser. The Mercury 12-191833 is an original equipment gasket designed for the 3/8-16 gearcase drain or fill screw. This composite washer, typically blue in color, provides a reliable, leak-free seal between the screw and the gearcase. It is widely used on many Mercury, Mariner, and MerCruiser outboards and stern drives, excluding Japanese-built models.. Key Features. Type: Composite gasket seal for gearcase drain/fill screw. Thread Size: 3/8-16 (standard for most Mercury gearcase drains). Color: Blue. OEM Quality: Genuine Mercury factory part for precise fit and performance. Why Use the Mercury 12-191833 Seal?. Exact Fit: Manufactured to Mercury's specifications for proper alignment and sealing. Marine-Rated: Built to withstand exposure to oil, water, and harsh marine environments. Essential Maintenance Part: Prevents water intrusion and gearcase dam