Originally Posted by jmraspa
Do these things come in different sizes, or what?
Yes, they do come in different sizes. Assuming we are talking about a 1" threadless headset, there are two common sizes. These are commonly referred to as "I.S.O." and "J.I.S" standards. The ISO will have a crown race with an inside diameter of 26.4mm and are commonly used in head tubes with an inside diameter of 30.2mm. JIS have crown races with a 27.0mm and are (usually) used in head tubes that are 30.0mm. The crown race sizes are not interchangeable, but the other diameter can be worked around (either you mash a 30.2 cup into a 30.0mm headtube, or you put a 30.0mm cup into a 30.2mm headtube and use a special locking compound). Your best bet is to take your bike to the LBS and let them do the measurements. Tange Seiki are very common headsets and your bike could be either ISO or JIS. You really have to do the measurements to be certain.