Our Advantage Zirconia Flap Discs are 4 1/2 inch diameter with a 1/2" thick with resin coated paper backing. The 7/8" arbor hole has a durable metal outer layer and is compatible with 4 1/2 inch grinders that have a retaining nut.  These zirconia flap discs are ISO 9001 certified with a 13,300 maximum RPM.  They are 60 grit which is a medium grit for light sanding polishing steel.