21.1(5)mm = .833" ---> This is a less common quill size typically found on old Schwinns, cheap cruiser-style bikes, and older BMX.
22.2mm = .875" ---> This is by far the most common quill size used on road and some mountain bikes. There's no easy way to interchange the two sizes. I'd recommend remeasuring before buying anything.
If you happen to be interested in a cleaner solution to going threadless than using a quill adapter I can probably help.