Quote:
Originally Posted by FastforaSlowGuy
I'm sourcing a new headset for an old CAAD frame, which takes the Campy-style headset bearings, which are 41.8 and 45*. A lot of headsets out there use 45* bearings and list their OD as 42. Is that an actual measurement, or is the market using 42 as shorthand for 41.8?
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42mm is the nominal size of the bearing. Because it is intended to be a slip fit, the bearing is slightly undersized. Here's a drawing of a FSA headset, showing that the top bearing is 41.8mm, to fit into a headtube bore of 42.0 +.1/-0.0.
Just last week there was thread about a press-fit crown race, where it was discussed that that the nominal size 30mm crown race was intentionally undersized (to ensure an interference fit). Depending on fit requirements, parts are frequently and intentionally made under or over their nominal sizes.