I have an old Windsor Carrera Sport that takes a .833" stem. The headset cups are a little pitted and I was thinking of replacing it. Is the outside of the steerer going to be the normal 1" size, or will I need an unusual headset?

Switch out the stem and headset to normal size and be done with it!

How old is "old?'

I believe late '70s. The left crank is stamped '76.

Oh... Why the odd fork then? My brain is in "early 1900's" mode, I was thinking that maybe it was one of those obscure things from the less-standardised days. Because I don't know much about windsors.

Pics?

Oops... I meant .833" (aka 21.1mm), not .883". Apparently it was used for a while by Schwinn, Windsor, and a few other brands.

I don't have pics handy, but the bike looks very similar to the one in this thread: https://www.bikeforums.net/classic-vintage/535818-windsor-carrera-sport-am-3-100-stock.html

Now that I'm searching on the right number, I found this thread, where someone claims that steer tubes for .833" stems have a 1" O.D., just with a thicker wall: https://www.bikeforums.net/classic-vintage/485638-21-15-quill-stems-sources.html

So I'm probably all set.