These bearings take 4.5 mm steel BBs. (Copper might work, but I have heard that they may cause problems due to the metal differences.)
I printed these on a Printrbot Simple in PLA at .25 layer height and 10% Infill.
(I have also tried to print these on a Makerbot Replicator 2 at the Low Resolution with 10% infill, but they had a much lower tolerance. Ie, they turned out very tight and it took a long time to wear the BBs in to their races before smooth travel was possible.)
Short: 23 mm High, 22 mm Outer Diameter
Long: 48 mm High, 22 mm Outer Diameter
Inner holes are 8 mm in diameter. (This should work on 5/16” rods as well.) After printing you may want to use an Exacto knife or 8 mm drill to make certain the hole will give clearance for the rod as the plastic tends to swell into it a bit when printed.)
The Combined Version is the same size as the Short with the addition of parts to allow it to be used on a Prusa i3 Rework Y Bed. When assembling the combined version I suggest placing the BBs in the races, placing the top and bottom on a flat surface and then using a clamp to press them together. (Attempting to twist them together may result in breaking the mounting brackets.)
The Short uses 16 BBs. The Long uses 32 BBs.
I really liked the concept of using BBs for the bearings like in http://www.thingiverse.com/thing:28905, but my attempts to print them out failed. I tried to use Netfabb Basic to repair the file, but still was ending up with 739 flipped triangles. So I began to design this version instead. One of the things I was hoping to do is be able to create a design that could allow me to modify in order to create odd sized linear bearings. Tearing apart inkjets and laser printers I have found many of the rods are 5, 7 and 9 mm sizes which are not typical linear bearing sizes. I’ve included the original FreeCAD model files in case you want to also modify the design to fit your needs.