Twinthread Countersunk Zinc Plated Screws For genral applications into soft wood and various types of hardwood. Designed & manufactured to BS 1210 standards - major diameter & threads excepted. 25º sharp point and single leading thread for a fast start and improved thread acceptance. 60º Twin-threads with parallel shank for faster insertion and greater pull out resistance. Surface hardened up to 750 HV to prevent torque shearing under load and a core hardness of up to 450 HV for improved ductility. Plated in 3-5 microns. Double countersunk reduces the likelihood of head shear and assist completion of countersinking.