Z axis coupler 8 to 5 mm.

thingiverse

I designed a custom coupler to replace standard aluminum ones. It was printed from ABS plastic. To assemble the coupler, print two copies of the central part and glue them together. Be sure to sand all parts thoroughly and apply lubricant to minimize friction. However, exercise caution not to overdo it with the sanding or lubrication process. M3 bolts and nuts are required for assembly.