The Firepower 1423-3186 Type 27 Depressed Center Grinding Wheel without Hub measures 4-1/2 Inch x 0.045-inch x 7/8-inch. Firepower depressed center wheels are fully reinforced with resin-bonded aluminum oxide to ensure safety at high speed and to increase bonding and impact strength. Designed for use on air- or electric-powered right angle or vertical shaft grinders. Maximum RPM: 13,300. Grade A36S.