Has a plastic "bumper" around the block to protect washing surfaces. Built with a single threaded hole. Grey flag are flagged plastic for fine dusting. Excellent water holding capacity and "flicking action". Heat resistant up to 190 degrees. Nylex truck brushes are Nylon 612 fiber is super light, resistant and durable. Can be boiled and is impervious to solvents, alkalis, certain acids. Do not use with mineral acid. Better water holding than other synthetics. Heads only. Use any threaded handle.