Martyr anodes provide low-cost marine protection for boats, marine structures, and boat equipment. Magnesium "Fresh" anodes are specifically designed for use in freshwater environments. Martyr anodes are made of a specialized alloy based on the U.S. Mil Spec using only high-purity metals. Weight: 4.8 oz. Dimensions: 3.6" O.D. x 3.3" H. Thread: 7/16 - 14 UNC. Model#: CM31640M. Dimensions: 3.6" O.D. x 3.3" H Thread: 7/16 - 14 UNC | Mercury Anode for 50 HP Engines - Magnesium