The ALM GH006 Sliding Door Wheel Kit is suitable for most greenhouses with sliding doors. The door can be re-drilled and the wheels re-positioned if existing wheels are different sizes. Contains: 2 x 28mm Nylon Wheels With Steel Bearings.2 x Aluminium Bolts, Nuts and Washers. Suitable for most greenhouse sliding doors. If your existing door wheels are a different size you can re-drill the Aluminium frame to re-position the door to still run at the same height.