Sierra 18-6241 Magnesium Trim Tab Anode for Mercruiser, Mercury Sierra high quality aftermarket marine parts are manufactured to meet or exceed OEM requirements. Magnesium trim tab anodes should be used in fresh water only and with aluminum props. For RMRAlpha One, Sterndrive Transom Assembly 1983-90 prior drives Alpha One, Gen II 1991 Up. Mercury Outboards Use on inline 3, 4 6 Cyl, V6 Replaces Mercury 31640 97-31640Q4 31640A1 316404 316403 31640A2 31640A4 31640B4 72436A1 Honda Outboard 4110