Allows you to adjust the tension on your y-axis belt by turning screws.
1) Print the casing, insert and pin.
2) You'll need to drill two new holes on the print bed about 12mm outside the two holes that are already there for the belt retainer. I found it was easiest to just clamp the casing into position and then drill up from underneath. Make sure that the belt pass-through hole is aligned with the belt retainer in the rear so that the belt runs in a straight line laterally.
3) Once you have the holes drilled, mount the casing using the two screws from the belt retainer that came with the kit.
3) Run the belt through the casing and mark where the belt comes to when pulled tight, inside the casing.
4) Remove the belt from the far end belt retainer.
5) Run the belt through the center hole in the insert, loop it around the belt pin, and back out through the center hole in the insert.
6) Make sure that the point you marked on the belt is on the inside edge of the insert, pull the belt tight around the belt pin and secure with a zip tie.
7) Run two 6-32 screws through the two adjustment holes on the insert and install hex nuts. You may want to tighten the nuts down so they seat in the hex inlays in the insert. (I tacked mine in with a bit of CA glue.) Set the screws so that they're flush with the hex nuts on the inside side of the insert.
8) Pull the belt through the back hole of the casing to bring the insert into the casing. The insert should rest flush with the back inside face of the casing.
9) Re-attach the belt to the far end belt retainer and secure with a zip tie. Make sure it is snug but not too tight.
10) Adjust the belt tightness by turning in the adjustment screws on the forward face of the insert. Test the tightness as you adjust the screws to get the right amount of tension. You want it to be nice and snug, but not so tight as to strain the components or to cause the Y-axis stepper motor to stall.