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Depends on the stem. I've been using 25.4mm bars in a 26mm Deda CrMo closed clamp stem ever since I built up my Pompetamine, but I wouldn't go near an aluminium stem of any design which has been thus abused, the material can't tolerate the strain and the thickness of the bit which gets bent means it actually has more strain for the same change of radius, compared with a thin walled CrMo or Ti clamp.
25.4mm bars in 26mm stem, likely to end in immediate certain death or fine?