[Ian Mackenzie]

I also like 20 pitch, although with SDP/SI selling a good selection of metric gears, I'm tempted to switch at some point. I like 20 pitch for the reasons everybody else has mentioned - they're common, quite strong, and not too heavy. However, another advantage of 20 pitch over something smaller is that they're less sensitive to small displacements - if for whatever reason the two shafts are not exactly the right distance from each other, larger-pitch gears are much less likely to get damaged or become inefficient.