Abstract: Parallel spaced rotary arbors support a plurality of axially spaced circular slitter knives, and a series of spacer rings precisely locate the slitter knives on each arbor. Each spacer ring includes a rigid carrier ring having three circumferentially spaced undercut cavities which confine corresponding floating gage blocks each having a precision axial width slightly greater than the axial width of the carrier ring. The carrier rings on each arbor are rotatably aligned by a keyway and include a pair of flat ring sections each having circumferentially spaced tapered holes which mate with corresponding chamfered surfaces on the gage blocks. The ring sections are secured together by screws or spot welds, and the wider spacer rings have corresponding core rings confined between the ring sections.