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I'm not sure if I'm missing something about IS disc tabs, but if you had a steel frame and cold set the dropouts to change the spacing from 142mm to 148mm, the IS disc tab would be in the correct position for a 148mm Boost hub, right?
Or is the offset from the hub endcaps to the IS tabs different for 142mm and 148mm dropouts?
Just to say, I'm not planning on doing this so please don't try to talk me out of it. It just occurred to me I don't know what the offsets are and for some reason I can't find a diagram that tells me.
Yes - same offset, which is how Surly make their Gnot_Boost 145 back ends work on either 142 or 148.
What he says ^. Stick a 142 wheel in the left dropout of a 148 frame and the disc should line up fine, shift it over to the right dropout and the cassette should be aligned to the derailleur.