Dia: 60mm (2.5in) By running your stove at optimum temperature you will avoid dangerous tar build up in your chimney, reduce creosote production and other emissions, and save yourself wood as burn time will be increased. Secures magnetically to the steel part of your flue and can be further secured either by the included safety screw or with a piece of wire. See what your stove is really doing and start reaping the benefits of optimum performance.