 Illinois 's main armored belt was 16 @.@ 5 in ( <unk> mm ) thick over the magazines and the machinery spaces and 4 in ( 102 mm ) elsewhere . The main battery gun turrets had 14 @-@ inch ( <unk> mm ) thick faces , and the supporting barbettes had 15 in ( 381 mm ) of armor plating on their exposed sides . Armor that was 6 in thick protected the secondary battery . The conning tower had 10 in ( 254 mm ) thick sides . 
