- headset sizing
- The nominal size of a headset is based on the outside diameter of the steerer. This is a source of confusion, because the steerer is not visible on an assembled bicycle. In the case of a bicycle with a traditional expander/wedge type stem, the stem shaft will be 1/8" smaller than the steerer. Sometimes people measure the stem diameter and assume, incorrectly, that this is the size headset they have
Dictionary of automotive terms. 2015.