STD ISO 239. UNF. Built for the most precise bores. The ergonomic self-tightening system allows for a very simple and quick operation. A clamping key is not necessary. Subtly knurled surfaces, electropolished. Round edges. All parts hardened for a max. chuck life. Ducts and all contact surfaces are ground. Checking for functioning and precision is done in accordance with standard ISO 10888. Through hole. Permanent markings, stamped. For clock-wise rotation.