I was unhappy with the 5/16 threaded rod that the Printrbot comes with and the Z-ribbing artifacts. The problem is the stepper motors have an uneven amount of steps with the 5/16 rod. When you print something with the 5/16 rod, your layer heights will be off every x number of layers unless you choose very specific layer heights. By going to 5mm threaded rod(mine cost $15 shipped from McMastercarr) you can change your steps from 2267.72 to an even 4000 steps. This gives a nice even layer height all the way through your print. Also, by adding dual nuts and a spring, any backlash is excluded. The other issue is the 5/16 rod coupled to the 5mm motor shaft is a poor design, by using 5mm threaded rod and some tubing, you get an even coupler and no z wobble.
This is hands down the best upgrade I have done on my printrbot, it improved my prints by a huge amount, if you do any upgrades, this is the one to do, best $15 bucks you will spend.
Print off the parts, one for each side, they will fit perfectly into the stock Printrbot Plus V2.
Cut the threaded rod to length and install nuts with spring in-between into the new part. Couple the threaded rod to the motor shafts with a short piece of tubing and zip ties from your local hardware store.
Adjust your Printrboard firmware in Repitier from the 2267.72 to 4000 steps on the Z-Axis and print!!!
Also, if you want the best, purchase brass 5mm nuts, however they only come in quantities of 100 from McMasterCarr for $15 so I didn't do this to mine, however, if you want the best possible setup, go with the brass instead of Stainless nuts.