hi there i have 2 bikes where i repeatably measure a 22.1 mm stem size?
Is that just a measurement error or does that size exist?

I know there is 22.0 mm and 22.2 mm, are they interchangable?

How are you measuring it? Could be measurement error. I don't know of any stem standard with that diameter.

22.0 mm and 22.2 mm are not interchangeable. I have seen standard 22.2 stems filed down so they can fit into an old French 22.0 headset, but this is not the best idea.

im using calipers, i measured all my other stems accurately and repeatably to 22.2 mm :S

A lot (think millions) of old Schwinns, some Ross bikes, some very old mountain bikes, and various old cheap bikes use 21.15mm quill stems. The stems themselves are usually stamped with the numbers .833, meaning .833 inch. The outside diameter of the steerer tube is one inch, just like steerer tubes that use 22.2mm stems, but the steerer tube's walls are thicker, which means the steerer has a smaller inside diameter.

Those sizes aren't interchangeable. The stems you measured at 22.1mm are probably a little undersize and meant for 22.2mm ID steerers. I've seen a few that don't fit precisely.

I misread the title and the OP. I'm tired. Sorry.

And lots (millions probably) of 1970s Nishikis and Centurions have the .833" / 21.1mm quills.

so I can use them in 22.2 and 22.1 mm stems interchangable since they are just poorly manufactured 22.2 mm stems??

Yes.