I went through my stash of stems at home, and all of the removable-face stems have some gap when installed.
Modern stems come in two primarily sizes 31.8, and 25.6, with 31.8 being the far more common, particularly on ‘LBS grade’ bikes.

There also exists 35 mm (D/H MTB) and 22.2 BMX, but all of those sizes are far enough apart that it is really obvious that you’ve got a mismatch. It’ll either rattle around in the clamp, or it won’t even go in, there’s no ‘almost’


*I’m sure someone will point out that 26.0 is really close to 25.6, but that hasn’t been used for almost 40 years, and then only in France, so it’s highly unlikely to find, unless you traffic heavily in imported vintage bikes