A z-offset spacer can be used when you want to add another plate on top of the existing build plate without changing the parameters for applying a z-offset in your slicer. Please note that making alterations like this is not covered by the machines warranty. Replicator_2-2X_4mm_z-offset_spacer (By CT3D.xyz).stl is 4mm thick and designed to fit both the MakerBot Replicator 2 and the Replicator 2X. Mount it on the rear plate on the Z-carriage to the left side of the Z-rod.