Joined: Jan 2010
Posts: 9,813
Likes: 1,790
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
[QUOTE=FBinNY;14623411]
So, for example if you bought a JIS fork for a bike that used to be ISO, all you'd need to replace is the crown race. (staying with the same brand/model)...QUOTE]
+1
I think that this is the only thing that the OP needs to know, but by now he/she must know at least enough to be dangerous!
One way to think of JIS is the raw frame, before any precision machining is done.
Cheaper bikes are sold that way, which usually works well enough with well-made headset parts.
ISO is like the same frame/fork with a little bit of metal machined away, to achieve precision-machined tolerances.
Sometimes JIS frames and forks are machined after the parts are welded or brazed, but you can't count on it, so distortion can cause fit issues in some cases.
Also, some brazed frames from the old days had ISO dimensions but actually lacked the post-brazing prep cutting that would assure exacting tolerances.