Originally Posted by
nafun
What does the 21.1mm measurement apply to? How did you measure it?
If i am wrong, and it is not a 1" steerer, then your options will be very limited.
Old Schwinn's, old BMX and some road bike forks have inside diameters that are 21.1mm on 1" threaded fork. The headset is also different.
I don't know much about the clamp diameter. You could try prying apart(big screw driver) the clamp on a 25.4 stem and see if you can make it work.
I doubt you find the combo you seek, 21.1 is old and front loading 26 is new