The hard floor castors set 6991 from Topstar include 5 castors. We recommend the double wheel castors for the use on hard floors (e.g. marble- and stone floors). The castors have a 10 mm-bolt and a diameter of 5 cm.     Castors for the use on hard floors (e.g. marble- and stone floors)    Hard floor castor with 10 mm-bolt    Hard floor castors with a diameter of 5 cmFeatures: -Hard floor castors (Set of 5).-Running tread made of thermoplastic grey rubber.-Double-wheel safety castors for hard floors: Tiles， stone， PVC， parquet， laminate.-No assembling required.-Distressed: No.-Country of Manufacture: Germany.Dimensions: -Overall Height - Top to Bottom: 5.5.-Overall Width - Side to Side: 5.-Overall Depth - Front to Back: 5.5.-Overall Product Weight: 0.48.Warranty: -2 Years warranty.