High-Low Self-Tapping Screws
High-Low self tapping screws have a double lead thread where one thread is high and the other low for easier penetration into the substrate. The high thread is sharper than a conventional thread. The sharp high thread causes less displacement of material when it is driven into the substrate. Consequently, it requires less effort to drive into material. In addition the greater amount of material remaining in between the high threads, the smaller minor diameter thread increases contact with the substrate resulting in a stronger grip and increased resistance to pull out and vibrational loosening forces.