Finally decided to upgraded the 5/16" threaded rods to lead screws on my Printrbot Plus LC. The rods are 4 start 2 mm pitch with 8 mm lead.
The adapters are tapped and secured to the nut with an M3 x 6 mm screw.
Update firmware DEFAULT_AXIS_STEPS_PER_UNIT to 400 for Z axis. Or M92 Z400 at a terminal and M500 to save it to EEPROM.