Jacobs Thread Mounted Chuck, Heavy Duty Jacobs thread mounted chucks feature jaws center-ground for absolute straightness and alignment and a one-piece sleeve that eliminates cracks between driving teeth often found in other designs. Jacobs thread mounted chucks feature through-hardened sleeve teeth, plus hardened nose and keyholes provide outstanding wear resistance. Each chuck 100% inspected for performance and precision. T.I.R. .004” maximum at half capacity.