The axle system of the Artech front and rear wheel are developed in such way that it ensures that the wheels run smoothly when installed in the bike without any adjustments. Since the through axles have a significant clamping force which could result in a tension on the bearings, the Artech hubs are designed with an intentional buffer. As a result, when the Artech wheels are not installed in a bike there is a slight movement possible on the axle system which can be explained as play. This movement is intentionally and will disappear whenever the wheels are installed in the bike and the trough axles are tightened according to specification.This kit is containing two different engagement spacer and O-ring kits, namely (i) the standard bronze engagement distance bush and the black O-ring kit, and (ii) the a special blue colored engagement distance bush and O-ring kit that should only be used in case you experience axial play on your Artech wheels when mounted in the bike.