Your first statement is right.
I cut where the red arrows are.
The thickness of the plate is 1/4". I started with 3/8" but the caliper ended up a little close to the disk. 1/4" worked out perfectly.
There is a person out of Canada that sells the kits for 10" rims. I origionally inquired about the kit before decing to make it myself.
Here is the email that was sent to me.
"Hi Sean,
You will get the caliper mounting plates that enables you to mount the
1984 - 1987 Honda Civic calipers and brake pads (which you still need
to buy on your own) and the Honda Civic rotors which i will provide
with all the machine work done if you buy the full kit he machine work
required for the rotors are :
Bore the centre of the disc out to 65.0mm diameter and Machine the
disc down to 7.90" diameter and Re-drill the disc accurately to suit
the Mini stud pattern.4 holes 10.0mm diameter on 4.0" (101.6mm) PCD).
The only thing that you must do is knock the studs out of your
existing drive flange. Set up true in lathe, machine the front of
flange down to 65.0mm diameter. Only take a few thou off the front
mounting face to clean up, where the drum sat and allow the rotor to
sit true. You'll need to get cooper S studs which are longer and a 1
inch spacers. all the things you'll need to get is:
1985 Honda Civic Calipers w/bolts & pads
1 inch spacers
two extra copper washers.
cooper S Studs
The offset is the same as a mini equipped with "S" brakes. The plates
fit onto your existing spindle (were the backing plate from the drum
brakes fit) if you wish to order the basic kit it will cost $300 Cdn.
plus shipping the full kit will cost $500.00 Cdn. plus shipping. This
kit will not fit if you are planning to use stock 10 inch steel wheels
( i know for sure that 10 inch steelies, dunlops & cosmics don't fit
but newer alloys like minilites do). Enclosed is a pic of the kit on
my mini (the spacer is a little larger (1.5 inch) but this was done on
purpose for racing applications) and the plate mounted on the spindle.
if you wish to order please give me you i hope this helps to clarifies
things for you. If you have any questions
please feel free to ask
Tamer"
The only thing that I dont understand is the use of the spacers, I didnt need it for my wheels, but maybe the 10" rims need to be spaced out to clear the calipers....I'm not sure.
Here is a link to his site.
http://joltfreak.tripod.com/
Cheers
-sean
p.s. Is there a turbo on that M3?