MGDuff Magnesium 1″ Shaft Anodes are designed for imperial sized shafts only because 1″ is 25.4mm in reality. For fresh water use.Shaft anode incorporating a mild steel core bar to control anode wastage and prevent the anode from shaking free as it is consumed. Specifications Drive shaft diameter: 1″ or 25.4mm Length: 55mm Width: 55mm Depth: 55mm