Type 27 Zirconia Alumina Depressed Center Wheels, 4 1/2 in, 7/8 in Arbor Hole Size/ DIAMETER: 0.875 INCHES NOMINAL MOUNTING OPENING SHAPE STYLE: 1 ROUND USAGE DESIGN: METALWORKING MAXIMUM SPEED RATING IN RPM: 13580.0 MOUNTING METHOD: SPECIAL FEATURES: 4-1/2IN. DIA. DEPRESSED HUB, 1/4IN. THICKNESS