Originally Posted by dddd

I have a cut off remnant of a 1" threadless steer tube that I use to verify clamp diameter, and also a center section cut from a bent 26mm handlebar.

That leaves just the 26.4mm diameter as the only remaining possibility, a size that corresponds to older Cinelli, Ambrosio and even some Shimano aero-era stems.

TTT's 25.8mm measures closer to 26mm than the numbers indicate, so I freely interchange 25.8mm parts with 26.0mm parts, same as we did in the shop 25-30 years ago.

Some claim that forcing a 26mm bar into a 25.4mm stem improves the slip resistance, and no doubt the bar diameter shrinks as the stem clamp is tightened, once you get the bolt threads started.

I'm recalling my Trek 720 coming with a 26.4mm Cinelli stem clamping a much smaller Beleri-Franco handlebar, and that setup never slipped either.