This is a mod that allows you to change the Z-axis threads (either imperial or standard metric) to trapezoidial threads (from https://hfbshop.de/, german only -- sorry). As those rods come with nuts that have a a 17mm wrench size you'll need this adapter to make it fit in the 13mm holes of the X-Z axis brackets. NOTE: Depending on what your previous rods were (e.g. M8x1.25), you'll need to adjust the steps per mm in your firmware, otherwise your objects will be too large in Z direction. M8x1.25 has 1.25mm Z per rotation while the Tr8x1.5 has 1.5mm. This also should have an effect on the layer height that you're using in your slicer. I usually use either 0.09, 0.12 or 0.21. These are the layer heights without Z rounding errors. Check Josef Prusa's calculor page. You must print this upside down! Duplicate it, you need 2 of them. You should use brim to print unless you have a very sticky build plate! It fits fairly tight but that's by intend. You don't want to introduce more gaps with this, otherwise you could run with the ootb rods :-)