Allows you to adjust the tension on your x-axis belt by turning screws.
1) Detach the x-axis belt ends from the extruder carriage.
2) Place the casing onto the extruder carriage so that the belt pass-through hole aligns with the one in the carriage. You may need some Proxxon/Dremel/sanding action to make it fit correctly.
3) Route the x-axis belt ends up through the carriage/casing assembly.
4) Open up the screw holes on the insert with a 5/32" drill. Insert a 3/4" 6-32 bole through the hole and attach a matching nut on the bolt. Snug the nut down so that it seats into the hex nut recess in the insert. I tacked mine on with a bit of CA glue as well.
5) Remove the 6-32 screws for now.
6) Place the insert into the casing and feed the x-axis belt ends though the insert and pull it snug. Push the insert all the way into the casing until it rests on the bottom.
7) Pull the x-axis belt taught, but not too tight, and insert the belt pin (or other shim material) between the two belt ends on top of the insert. Secure the belt ends with zip ties. I used one just above the belt pin and another around the belt pin itself.
8) Reinstall the 6-32 screws. Adjust the belt tension so that it is the proper tightness... nice and snug but not so tight as to put excessive strain on the system components, or to stall the x-axis stepper motor. Somewhere around 1/4" deflection on the belt before you're really tugging on it.