10 Advantage Zirconia Flap Discs that are 4 1/2 inch diameter.  The discs are 1/2 inch thick with resin coated paper backing. The 7/8 inch arbor hole has a durable metal outer layer.  They are type 27 zirconia flap discs with a 13,300 maximum RPM.  ISO 9001 Certified.  The 40 grit is considered very coarse for grinding and sanding steel.