can please someone explain me how many sizes of quill stems ? in mm pls. i am restoring an old mtb, and noticed that the inner of the fork is 21.1 mm. is this a rare size?

thanks

No, 21.1 is not a rare size. It is most commonly used on dept store cheapies and older schwinns. Finding a decent quality 21.1 is a bit more challenging, however. But they are out there.

Some old mtb's used this size, although I don't think much logic was involved. The original "mountain bikes" were derived from Schwinn balloon tire bikes, which used this stem size, so apparently that is the reason.

Do you think a 21.15 stem will fit?
https://harriscyclery.net/itemlist.cfm?category=241

Is this a ross MTB

SWo far as I know 21.15mm and 21.1mm are the same basic stem size, just different roundings of the number. The difference is less than .002".