This brush is made from .050" Prostran bristles which are more flexible than our heavier Poly Pro brushes. It is ideal for applications where greater maneuverability is needed or for use in prefab fireplaces allowing the brush to more easily pass through the damper. Also a great choice where creosote build-up is such that a regular brush is too stiff to pass down the chimney. Bristles will not harm metal chimney pipe. The spindle has a 3/4" loop on one end and 3/8" Male NPT thread on the other.Note: If going thru a 45 degree or more elbow, use ProFlex Rods and a female to female adaptor.