Originally Posted by
soyabean
So that means caliper measurements only help you guess to what it could be, but ultimately you have to get the piece to see if it fits
It's an electroforged Schwinn. They made millions over decades. The quill stem will be a .833 and the seatpost is 13/16" (20.6)
The OP is in Boston. I'm sure there has to be a co op there that has a stem for $10 or less.