Like its predecessors, this Tinkertoy-compatible bearing system uses ten 4.5mm BB's to provide smoother rotation than the stock wheels included with a Tinkertoy set: http://bit.ly/1ngex61The center bore is *just* a little loose on a Tinkertoy rod (not renowned for their precision diameters in the first place) so if you really need the bearing to stick fast I recommend using a little piece of painters' tape around the rod. It's modeled in three parts for ease of assembly, but you'll need to super glue the two halves together.