FC 390-521C 31" 18 Single Row .035"/180 Mid Grit Abrasive Cylinder Sweep/Scrub Brush for Factory Cat 390 Riding Floor Scrubbers. A middle grit scrub brush for general duty scrubbing tasks. Narrow .035" diamater Tynex-A Nylon filament impregnated with 180 grit silicon carbide granules for reaching medium and small cracks and into grout lines. Equivalent to Midi-Grit. Usually blue or grey in color. 31"L x 9"OD. Fits Factory Cat 390-C cylindrical scrub brush riding scrubbers. Some units (390, Disc) require two 20" disc brushes instead of cylinder brushes (use two 20-421C Midi-Grit equivalent - 19" block diameter). Please check your unit before ordering. Two cylindrical brushes create a 36" wide scrubbing path. Two brushes required. Priced each. Replaces Factory Cat 390-521C