Aluminum Anode for Mercury Mercruiser Alpha 1 and Most Outboards Standard Trim Tab Skeg Replaces 31640 Replaces 31640 Aluminum Anode For Mercury / Mercruiser Standard Trim Tab Fits Most Mercury/Mariner Outboards 35 hp And Above. Fits Mercruiser Alpha 1 Gen 1 Outdrives. Base Diameter: 3-1/2" Height: 2-1/2" Threaded Mounting Hole: 7/16"-14 This Is A US Military Grace Aluminum Anode Made To The Highest Quality. Aluminum Is For Salt And Brackish Water Use. Replaces: Mercury 31640Q 4, Martyr CM31640A, Honda 4110-ZW1-003ZA, Quicksilver 31640A, Tecnoseal 00800AL, Camp 31640AL