The frame spacer is the part of the frame that holds the wheel. For plastic frames it is often replaceable. This specific part is used in K2 skate frames, it will not fit in any other brand then K2.

The special thing about these spacers is that they have a HEX shape

The inner diameter for this spacer is 6mm, because that is also the diameter for the axle.

The sides of the HEX are 10,8mm apart

This spacer is sold per piece.