The toothed side of the half bush can be meshed with a
number of other LEGO pieces: another toothed half bush,
a toothed axle connector, or a toothed 16-tooth gear with
clutch (as shown in Figure 4-9). It’s important to keep in
mind that when these pieces are meshed, they come roughly
1 mm closer to each other, which means that there is 1 mm
of backlash behind them.