wrote:
Nashbar has a good deal on ultegra brake calipers; I mounted them on
my front wheel. My rear caliper is kinda corroded, and I was
considering mounting another front caliper on the rear wheel... Is
this a good idea?


as pointed out by others, the bolt length thing is not a big deal.
however, you need to be aware that shimano & campy dual pivot brake
calipers have a pivot action that increases toe on the pad as the
caliper closes. [easily observed for yourself if you have the caliper
in your hand - simply squeeze and note how the relative angles of the
pads change as the lever arms move through their arc.] if a front
caliper is used on the rear, the toe is towards the rear of the pads,
not the front, relative to rim direction.

will you notice any effect in practice? maybe some brake squeal, but
probably not. is it correct in theory? no. and iirc, there's a
warning in the shimano literature to the effect that each caliper is
specific to front or rear application and should not be interchanged.