Headset bearings are a mine-field.  Our research has pulled up well over 40 different sizes and types, some common and some bike specific (looking at you Trek and Cannondale!).

To make headset identification as easy as possible we've created a Headset Bearing Identification Chart.  We've even made measuring headset bearings simpler.  So grab your verniers and a credit card or business card (yes seriously) and hit the link above.