I spoke to a serious framebuilder about doing something analogous (130mm wheel on rear dropouts with 126mm spacing) and he was adamant that it was a bad idea -- you risk cracking a chainstay after a few thousand miles. It costs very little for a framebuilder to 'spread' the existing rear dropouts to accommodate a larger size. 126 to 130 was said to be easy; I have no direct experience with 120 to 126.
Hope this helps.
On Sunday, June 8, 2003, at 10:56 AM,
> Date: Sun, 8 Jun 2003 10:40:20 -0700
> From: "Greg Thies" <firstname.lastname@example.org>
> To: <email@example.com>
> Subject: [CR]126 into 120
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> Message: 14
> I would like to use a rear wheel that has a 126 mm axle on a steel
> frame =
> that has 120 mm spacing. Is this advisable? Are there any long-term =
> structural concerns? Thank you.
> Greg Thies
> Vienna, VA... where a new rain forest is in the making