Fresh Water Skeg Anode for Mercruiser Alpha 1 Gen 1 and Many Mercury Outboard Motors For Freshwater Use Replaces: Mercury 31640M, Martyr CM31640M, Sierra 18-6011M, 41107-ZW1-003M, Tecnoseal 00800MG For Mercruiser Alpha I Gen I Trim Tab Skeg Anode For Fresh Water 31640 Mercury/Mercruiser Standard Trim Tab Anode Base Diameter: 3-1/2" Height: 2-1/2" Threaded Mounting Hole: 7/16"-14 Fits most Mercury/Mariner outboards 35 hp and above. Fits Mercruiser Alpha One outdrives. This Is A US Military Grade Anode Made To The Highest Quality. For Freshwater Use. Replaces: Mercury 31640M, Martyr CM31640M, Sierra 18-6011M, 41107-ZW1-003M, Tecnoseal 00800MG