Boiler Tube Cleaning Brushes are supplied in Nylon, Brass, Stainless Steel or in Carbon Steel. Depending on the deposits within the tubes will dictate the brush medium. As for the brush size, you need the brush to be slightly less than the diameter of the inside of the tube. This then allows the brush to vibrate while rotating and cleaning. Brushes need to be married to the right tube cleaning machines. We can advise you on the correct machine to give you power needed due to tube size and deposit type. Alternatively, we rent these machines where you could try before you buy.