Roller material: solid zinc alloy with plastic shell & wheel diameter: 23mm & wheel thickness: 5mm Attention before installation: A. Suitable for 4-6mm thick glass B. The diameter of the glass opening should be greater than 12mm. 23mm diameter wheels can be placed in the shower enclosure D track. The opening spacing of the top wheel to the top is less than 12mm To fit the maximum glass thickness: 4-6mm and to fit the minimum opening hole size in the glass: greater than 12mm. For glass thickness greater than 4mm, we suggest that it is not necessary to use the gray plastic washer for assembly, because this plastic washer will not have enough space to fix it firmly. DIY Replacement: It is easy and convenient for this replacement door knob to DIY, you can simply disassemble it with a screwdriver and replace it with new rollers on the glass doors. Package Content: 4 x Shower Door Rollers (2 x Upper Rollers and 2 x Lower Rollers).