UPDATE 2015-11-11: Though this design works it's now been replaced by http://www.thingiverse.com/thing:1122480This helps remove Z ribbing caused by bent Z screws and/or Z couplers that aren't perfectly centered (Z couplers with non-centered holes is my problem). The before/after pics show that these certainly work and the Z ribbing is now virtually non-existent for 0.3mm layer height for me now. I've included tolerance in the design for the nut (13mm). The exact distance from flat to flat in the design is 13.2mm. The inside gap of the guide arms is 8mm. Make sure the nut fits firmly and the guide arms don't have any slop on the 8mm Z guide rod but isn't tight. I lubricated the inside of the guide arms and the flat of the Z isolator that the X carriage sits on. Take your X carriage and Z nuts off (which should be able to be used in the Z isolators) and manually screw the isolators on, flat side up, onto the Z screws by turning the Z screw by hand until they're about an inch odd down on the Z guide rod as shown in the 2nd pic. Do not try use the motor to do this as the Z screw will flop around and you could damage your Z coupler. Once you have both isolators on and at the same height put the X carriage back on and control the motors to lower the carriage. You'll need to lower your Z end stop and lengthen the screw, if you have one. Re-level the bed and do a test print.