20mm stem?
 

I have an old Schwinn Hollywood, with a badly rusted and cracked stem. At the headset locknut, it measures 19.8; an inch or two up, it measures 20.8mm.

Will a 21mm stem fit? I'm sort of wondering if all of the rust and cracks have made the stem narrower, and I"ll b ok with the 21mm (I'm replacing the headset too, which also is a goner).

thanks!