Since I have the UCF10, 11 and 21 brakes right now i thought i'd measure them up to see the differences.
UCF21:
Front piston: 1.54"
Hub to Piston: 5.0625"
Rear piston: 1.48"
UCF11:
Front piston: 1.38"
Hub to Piston: 5.0625"
Rear piston: 1.48"
UCF10:
Front piston: 1.38"
Hub to Piston: 4.625"
Rear piston: 1.48"
The only thing that changed between the UCF10 and 11 brakes is the distance the caliper is from the hub center. This can be seen in the carrier bracket. Which would mean a higher Force x Length for the UCF11.
You have disc diameters too? So I could buy just the discs and brackets to upgrade my 91' rears to the later model? that's good to know. I did the 95 front upgrade (really should do the front hubs too), but nothing to the rears yet, and I'm working them hard. I go through $400-500 in brakes every weekend I drive it!