Originally Posted by
steine13
And the first thing I learned about picking parts is that there are (of course!) two standards for the diameter of the collar that the crown race is pressed on -- 26.4 mm and 27.0 mm. Because that's how we do in the bicycle world.
That only applies to 1" headsets which come in two sizes, JIS (27.0 mm crown race) and ISO (26.4mm crown race). The OP has a 1-1/8" headset and they are all the same diameter.
Also, the crown race (or base plate in the case of a cartridge bearing headset) is usually a firm drive fit and will not seat fully without some force. There are specific tools, basically slide hammers, used to drive them fully into place. A few models of headsets come with split crown races/base plates that can be fully seated by hand. Velo Orange's Gran Cru headset is one that does. Most others require tools.