Nickel free. Extra Small: Cap 1/4" (6 mm), Post 3/16" (5 mm), Base 1/4" (6 mm)Small: Cap 1/4" (6 mm), Post 1/4" (6 mm), Base 1/4" (6 mm)Medium: Cap 5/16" (8 mm), Post 5/16" (8 mm), Base 5/16" (8 mm)Large: Cap 5/16" (8 mm), Post 1/2" (13 mm), Base 5/16" (8 mm)Steel. When only the best will do, these rivets offer the finished look that only double cap rivets can. Use setter #8100-00 and anvil #8056-00 or setter kit #8105-00.