Originally Posted by
BeeSeeBee
I'm helping a friend build up two frames for him and his girlfriend, both early 80's frames, his is a Cooper, hers is a Waterford.....
I'm not too clear on what each of the two standards actually differentiate, is it the inner and outer cup diameters? JIS seems to be 27.0mm and 30.0mm respectively, and ISO seems to be 30.2mm and 26.4mm respectively, but I'm not sure if frames are sensitive to that level of precision. Thanks for any help you guys can provide, the rest of the build is going quite fantastically
Odds are very strong that both the Cooper and Waterford are ISO standard for both the frame and fork. JIS (Japan Industrial Standard) is pretty much limited ro frames and forks produced in Japan for their domestic market. For a while a number of JIS bikes were exported to the USA, the last ones being Specialized.
Certainly a careful measurement of your existing headsets, or the fork crown to confirm is warranted. Measuring heat tubes to .2mm accuracy is a bit more difficult due to ovalizing so I'd pass on that.
As for whether it makes a difference, absolutely it does. There's no way an ISO crown will fit a JIS fork without shattering. In some cases you might fit an ISO heat cup into a Steel or Ti frame, but you'll risk splitting the tube, (with aluminum or carbon a split is virtually assured).
Lastly if for some odd reason your bikes are JIS, these are easily convertible to ISO, but the reverse isn't true.