We also sell out Advantage Zirconia Flap Discs individually.  They are a great value for the price.  These 4 1/2 inch diameter fit small angle grinders.   The zirconia flap wheel is 1/2" thick with resin coated paper backing. The 7/8" arbor hole has a durable metal outer layer and is standard for 4 1/2 inch grinders.  Advantage zirconia flap wheels are ISO 9001 certified and have a 13,300 maximum RPM.  40 grit is a coarse grit for grinding and sanding of steel.