My hinged 3ttt stem seems to have one of those 25.8 diameter clamps. A 25.4 bar I had looked a bit loose on the clamp's halves while a 26.0 bar was a very tight fit. Before tightening the 26.0 bar, you could se a very tiny gap between the bar and the deepest part of each half. Tightened up fine and the bars are really scratched up already so I didn't care.