Lego Gears

20 degrees         pressure angle
8/pi (2.54648)     circular pitch

http://blog.makezine.com/2010/06/28/make-your-own-gears/

http://www.robertcailliau.eu/Lego/Dimensions/zMeasurements-en.xhtml
http://news.lugnet.com/robotics/?n=21405&t=f&v=a
http://vector.us/browse/330348/lego_gear_profiles
http://www.lugnet.com/

http://www.notcot.com/archives/2012/03/free-universal-construction-ki.php