Use to extract cores of wood from trees or freshly treated poles, piling, lumber, or timbers to determine growth rate, age, tree soundness, and chemical penetration. Teflon® coating protects bit against rust and resin build-up and reduces friction. How to select the correct Increment Borer for your needs. Three factors to consider when ordering an increment borer are bit length, core diameter and thread style. Borer Bit Length depends on the size of the trees you will be boring. Length is measured from the tip of the threads to the end of the round section of the borer bit. This is the maximum depth the bit will penetrate. Core Diameter of the wood sample is determined by the inside diameter of the opening at the threaded end of the bit. 0.169" is commonly used for general forestry use, 0.200" for wood preserving testing and 0.500" for large amounts of wood for qualitative analysis. 2- or 3-Thread style is a matter of personal preference. A 2-thread borer has two threads on the cutting edge of the bit, each originating 180° apart. A 3-thread borer has three threads, each originating 120° apart. The higher thread pitch of the 3-thread borer will penetrate deeper per revolution than a 2-thread. A 3-thread will also produce less friction because more steel is in contact with the wood. The ease at which a borer penetrates wood depends on wood hardness, friction properties and capability/strength of the user.