This set of spur gears is intended as a training tool for spur gear theory. Fundamental gear parameters (in inches) diametral pitch (DP) = 3 inch number of gear teeth (N) = 12 pressure angle (phi) = 20 degrees then using standard spur gear equations: pitch diameter (PD) = 4 inch outer diameter (OD) = 4.666... inch Note that the units above are provided in inches however the .stl files are provided in millimeters. references: a .pdf from Boston Gear discussing both spur (& other) gear theory: http://www.bostongear.com/pdf/gear_theory.pdf print two of each part: gear_4_DP3_N18_150116a.stl gear_4_DP3_N18_150117_link.stl use care when gluing the parts together to avoid binding the gears.