Unslotted, Indented Hex Washer, Thread Cutting Screws, Type 23 Thread. Thread-Cutting Type-23 screws provide excellent chip clearance with minimum tightening torques. Steel: Used in cast iron and zinc, aluminum die castings, and plastics. Stainless: Material into which the screw is to cut threads should have a lower hardness. Indented hex washer head offers advantages of hex head, plus greater protection to mating surface.