Summary

Replacing the stock 5/16" with an M8 threaded rod is easy, as the tolerances are lower than the difference in width between the two rods. However, when upgrading to an anti-backlash setup with two M8 nuts and a spring, this requires a hexagonal tube which captures both nuts.

Instructions

This was inspired by the model made by Brian Evans for an M6 threaded rod, however this idea does not work with an M8 rod without doing some much larger alterations to the z/y carriage.

The "missing sides" of the hexagonal tube are to avoid the y-axis motor.

The fit of the nut and top edges of this piece is very snug and holds everything in place quite well. I haven't seen any problems with torsion due to the lack of two of the walls of the hexagonal tube.

Brass is the material of choice for threaded rods and nuts as brass-on-brass mechanisms are self-lubricating. Seriously, when you put a brass nut on a brass rod nothing feels more mechanically "right".

This upgrade is really cheap, at around Â£8 delivered in the UK.

Materials
M8 brass threaded rod.
M8 brass nuts.
10 x 32 mm compression spring.

However, after I had ordered the threaded rod and nuts I found the much more appropriate (and cheaper) combo of both, plus brass washers.

Printrbot Jr M8 Z-axis upgrade