having just upgraded the front brakes on my AE85 to AE86 items, i took the oppurtunity to take some measurements of calipers / pistons etc, hopefully this will provide a reference for people thinking about doing the same. i know its a pretty basic conversion but i struggled to find much info on it (maybe because its so simple?)
calipers
AE86 calipers are wider than their AE85 counterparts to accomodate the thicker vented rotors. piston size is 45mm diameter, compared to 42mm on the AE85. the AE86 caliper will slide onto the AE85 hanger, however the bottom slide does not line up. the AE86 caliper hanger is wider (naturally) and the bolt spacing that attaches it to the hub is different to the AE85 - so you will need the AE86 backing plate, and i would reccomend using the AE86 bolts also, since the backing plate is thicker and the AE85 versions barely thread into the carrier.
rotors
AE86 rotors are vented, compared to the AE85s solid discs. in addition the AE86 rotors are 235mm diameter, while the AE85s are 230mm. the disc carrier and wheel bearing setup is identical.
brake pads
the AE86 uses slightly larger brakepads, so replacing these is a must also.
minimum requirements
for this conversion you will need a minimum of -
AE86 calipers and matching hanger
AE86 backing plates
AE86 rotors
AE86 brake pads
AE86 hanger bolts
all the other stuff can be reused from your AE85. overall i would say the upgrade is a marginal one, particularly for the dollar these parts are commanding nowadays - i was pleased with the increase in stopping power, and i wanted vented rotors as i am hoping to get to a few track days at some stage, but i believe that bang for buck (for street use at least) you'd probably do better to invest in some new rotors and a nice set of performance brake pads unless you can get the AE86 stuff for around the $300 mark.