Originally Posted by
himespau
..If I go with a new bar, I'm pretty sure I need 25.4 mm. I'm not 100% sure though. If it turns out that I needed the 26.0 mm bar, I can buy a shim to make up the difference, right?
If your stem is 26.0 and your bar is 25.4, a strip out of a soda can is your friend.
Originally Posted by
himespau
.. Obviously there's nothing I can do if I need to go the other way.
If your stem is 25.4 and your bar is 26.0 there's nothing reversible you can do.
But you can wrap a coarse sand paper wrapped around a broom handle, insert into stem and get turning. If you get the fit close enough this can enlarge the hole enough while still keeping nice and round within the tolerances required for a bicycle.