Stem quills, for threaded headsets come in 3 sizes: 25.4 mm (1 inch), 2.22 mm (7/8") and 21.1mm (smallest, not sure of inches, don't want to do math now)
This must not be confused with the fork steer tube diameter which is 1/8 inch larger- therefore a 1 inch stem quill fits in a 1 1/8 inch steer tube, a 2.22 mm 7/8" quill fits in a 1 inch fork steer tube- (Headsets are described as the fork tube they fit: a 1 inch headset fits a one inch steer tube, but takes a 7/8 inch stem to slide into the inside diameter)
22.2 (7/8") is the most common stem quill that fits a 1 inch steer tube and 1 inch headset,
but yes, most newer mountain bikes now have the larger 25.4 stem, which fits their 1 1/8 inch fork steer and they have a 1 1/8 " headset- Road bikes went from 22.2 in the 60's and early 70's to 25.4 when Campagnolo went to a wider stem quill.
After getting familiar with the sizes, I can usually tell on sight- most stems are stamped with their size doown on the quill-
the small diameter 2.11 quill is the more rare and usually on older bikes, but quite standard in BMX old school bikes as well- not really indigenous to Schwinn, but Schwinn did have them on a lot of their bikes when others had largely gone to 2.22 mm.
And all this then must not be confused with handlebar diameter- usually just one of two sizes, again 22.2 (7/8 inch) or 25.4 (1 inch)- if a handlebar is thicker at the stem point of contact, it is usually 1 inch, if it is the same diameter as the rest of the bar it is 7/8 inches. Most handlebars are 1 inch at the center, excet for BMX which is normally 7/8 inch.
smaller sizes of all this can usually be shimmed to the next larger size up with shims, cutt aluminum or tape, but obviously, the larger never can fit the smaller size
