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Discussion Starter · #1 ·
Preface / Disclaimer: I got all of these ideas from OpelGT.com. This is what is on my car. We raced this car in the 24 hours of Lemons in Houston on September 11th, and our brakes could not have been better.
 

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Discussion Starter · #3 ·
The calipers I used were remanufactured. The first side i did bolted up easily and everything was lined up down the center of the disc. The second one was not quite as lined up. A few seconds with a disc grinder on the mounding pads of the caliper and it lined up as nicely as the first.
 

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A big thanks to you
 

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Discussion Starter · #6 ·
For the rear we removed all of the old hardware and drums. Then cut and ground the old flange off. I welded the brackets to the axle with everything assembled. I am sure that was not the best way to do it, but it worked for me.

I got the .pdf of the GM metric brake bracket off of another post on this forum.

This is a 1973 rear axle, the car is a 1970. Lucky me they came as a pair.
 

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Discussion Starter · #7 ·
Weld the bracket to the axle housing where the conical section meets the large diameter tube at the end. Make it as square as possible. Ours was not perfect, but pretty close. Some bending with very large vice grips got us corrected.
 

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Discussion Starter · #8 ·
We ran new 3/16" tubing all around the front. The solid line and the tee on the rear axle are the only original lines. At first we left the original rubber lines to the front calipers. After attempting to bleed them for an hour we determined that they were plugged. I searched the web and found Techna-Fit out of California. They had the best price and I really liked that they make and sell their own lines and fittings.

There are reasons for going back and forth from inch units to metric and having a few an fittings as well. At first we tried to keep some of the original and use some new. Then we ended up replacing almost all of it. Also the tee fittings that I found at Auto Zone only came in inch units.

We tried using the 22mm bore Opel master cylinder from OGTS and could never get it to bleed. We tried for a long time and finally decided it was not going to work. We tried the original MC which was a bit better but is not idea given the addition of rear discs.

The BMW master cylinder gave us a bit of a pain, but finally we got it to bleed fully. It did not come with the rubber gromets that retain the resevoir. Luckly the Opel gromet flipped upside down fit into the BMW cavity very well.
 

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Discussion Starter · #9 ·
Caution. If you do this setup and your car is dropped two inches. Use the rear brake line with the metal part nearest the banjo fitting bent down.

Toward the end of the first day one of our rear rubber (new) brake lines gave way. We checked the other side and it was not far from failure. They had been rubbing on the body durring cornering and wearing the cover away. Use a brake line with a 90° bend down right after the banjo bolt hole.
 

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Discussion Starter · #11 ·
15" Steel wheels with 4 x 100 bolt pattern. I believe they were off a miata. 205/40R15 Nitto Neo Gen tires. We used 1" spacers to get the stance right.
 

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Discussion Starter · #13 ·
Note about the master cylinder: the ports are on the driver side. We did not have headlights so it was not a problem. To use this on a street legal car you could use banjo bolts to connect to the master cylinder.
 

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The Honda calipers need to have the id opened up a bit, and a new bolt pattern drilled. After that they bolt right up to the factory hubs.
You might want to do it the other way. You machine down the hub to 66mm to allow the stock rotor to be mounted. If you need to replace a rotor, just drill 4 holes and you are set to go.
 

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We ran new 3/16" tubing all around the front. The solid line and the tee on the rear axle are the only original lines. At first we left the original rubber lines to the front calipers. After attempting to bleed them for an hour we determined that they were plugged. I searched the web and found Techna-Fit out of California. They had the best price and I really liked that they make and sell their own lines and fittings.

There are reasons for going back and forth from inch units to metric and having a few an fittings as well. At first we tried to keep some of the original and use some new. Then we ended up replacing almost all of it. Also the tee fittings that I found at Auto Zone only came in inch units.

We tried using the 22mm bore Opel master cylinder from OGTS and could never get it to bleed. We tried for a long time and finally decided it was not going to work. We tried the original MC which was a bit better but is not idea given the addition of rear discs.

The BMW master cylinder gave us a bit of a pain, but finally we got it to bleed fully. It did not come with the rubber gromets that retain the resevoir. Luckly the Opel gromet flipped upside down fit into the BMW cavity very well.
To cover the compliance required for this type of system, you would need a Honda master cylinder (15/16").
 

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Just a couple of additional notes:
1) For racing, you might want to use Howe calipers in the rear; which are the same but made of aluminum.
2) In sizing a master cylinder for the 4-disk setup, you want to increase fluid delivery while maintaining a decent pressure. For this task, a Honda 15/16” master cylinder will give you 0.35” additional travel at almost the same pressure as the stock master cylinder.
3) Since the rear calipers are made for a 1” rotor and the Prelude rotors are 0.81”, you can cut down pedal travel by inserting a ¼” piston spacer. This spacer brings the pads closer, reduces the mechanical move to engage and therefore requires less fluid to engage.
4) Use ABS Volvo calipers for the front. They require only a single line and they are almost 1 Lb less in weight.
5) Locating the caliper bracket in the rear should be done with the caliper installed on the rotor and clamped. You the tack the bracket in place, remove the additional components and then fully weld in place.
6) The rear hoses should be from an ’88 Acura coupe. They get gently or loosely coiled on the axle housing and directly mate to the 10.0mm tube nut. This helps a lot if your car I lowered or making sharp turns.
7) The front hose, either standard or ABS caliper, are from a 1966 VW bus. The hose end has a 10.0mm IF fitting which directly screws into the caliper. You can run dual hoses to the bulkhead and make your Tee connections inside the engine bay.

I have used this setup on Opels for many years and have done a few adjustments along the way. I no longer use metric calipers for the rear, I now use Acura calipers with E-brake which are almost a bolt on scenario and can be installed with no welding. The front brakes were updated as well, using Cavalier calipers and Nissan Stanza vented rotors. This new front setup allows you to use the original Opel 13” wheels.

I hope this will help you on your next race.
 

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Thanks for the detailed description and listing the parts and prices. This is very valuable for anyone who would like to do this mod.:veryhappy
 

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Discussion Starter · #19 ·
Thanks blancojp, those are some very usefull hints. The spacer for the rears is especially useful.

The setup we have will probably stay as is, but I am thinking about building our parts car into another racer. If I do, your suggestions will help improve this setup.
 
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