Steerer tube INTERNAL diameter 1 1/8" (and thus size=OD=1 1/4"):


Quill circumference 89mm (--> OD=1 1/8")


I found the model, it was hidden behind a sticker, it is a Gary Fisher Advance:




and looking very carefully at the headset, you can just make out "Fisher Evolution"




Bikepedia.com skips from 1993 to 2003 for the Gary Fisher Advance, and the 1993 doesn't list headset info. But mine's not a 93, because that lists an Altus group, and this has Exage. For other models I can see 1 1/4" headsets up to 1995, then suddenly in 1996 everything seems to drop back down to 1 1/8.

Also FYI the too-long reach which caused this whole investigation is 110mm. But given the rarity of 1 1/4" headsets, I don't expect to find a replacement stem (for a price I'd be willing to pay for a $5 bike). My plan is to maybe look for some moustache bars or lean back some bmx riser bars or something.

Thanks all for your help (especially LesterPup, the only one who believed in me!)

BTW, I still don't think I've heard any answers to my original question, would it work if I chopped up the existing quill to use as an adaptor sleeve for an easy-to-find 1 1/8" stem?