Have you actually checked the bike to know it needs 21.1 mm? I know you probably wouldn't be here asking if you hadn't, but I have to ask.

While most Schwinns did use them I know that some didn't, like my 1986 Voyageur, which uses a standard 1" quill stem.

In my case, it is because it is Columbus Tenax... From what I was told at the LBS, Columbus refused to use the non-standard steerer diameter that Schwinn usually used.

Of course, he also said that all Columbus framed Schwinns were US built, so I was a little skeptical because I checked the labels and my bike is Japanese built. Then I slid my Technomic into the steerer for a perfect fit...