The frame spacer is the part of the frame that holds the wheel. For plastic frames it is often replaceable. This specific parts is made for the USD Transformer frames, but can also fit other plastic frames if it has the right dimensions:
The inner diameter for this spacer is 8mm, because that is also the diameter for the axle.
The outer square part of this spacer is 12,9mm X 12,9mm.
This product is sold per piece.