This turn buckle is fully functional, once threads have been turned. I use a 5/16ths tap and dye.

Be sure to print the turnbuckle itself, in the vertical position. This will ensure that you wont have to use supports to print.

When cutting the threads on the eye bolt, be sure to only cut a few threads at once, then back off, then cut some more, then back off. To much torque will destroy the part. You may want to use some sort of organic oil such as olive or coconut as a lubricant.

On the turnbuckle itself, you may have to increase the diameter a bit due to printers not being completly precise. Just make sure you are not using a bit bigger than the tap threads

I printed with the following settings for a davinci jr 1.0 NON PRO using xyz pla clear blue:

Good quality (default for normal)
10% infill
normal thickness for shells
0.3mm for layer height
standard speed(not given in mm/ps)
217*C