Abstract: An adapter ring construction and cooperative arrangement with respect to an arbor for installing tooling on arbors or shafts such as the rotary disc shear knives which are mounted on arbors of slitters for strip metal slitting lines which are operated to continuously slit multiple strands from a wider continuously moving strip such as strip steel. The adapter comprises a ring-like member which is threaded onto a reduced threaded arbor stub shaft of each slitter arbor, after removal of clamping nuts from the stub shafts and retraction of the outboard bearing housing for a pair of cooperating parallel rotary shear knife mounting arbors. The ring-like adapter has a special outer annular contour in cross section which facilitates easy, rapid and damage-free telescoping of rotary shear knives and spacers onto the arbor for a changeover of desired spacing of the rotary knives when the desired strand widths of multiple strands to be slit are changed.