G-body C5/C6 rear disc conversion


Sep 18, 2005
I recently finished converting my H/O with 8.5” 10 bolt to C5/C6 brakes, using a KORE3 kit with parking brake. This was not a bolt-on kit, but thanks to a ton of help from Tobin and some friends, we made it work. I figured I’d post this project up to describe the “why” and “how” for anybody else interested. My rear end was already converted to 30 spline Moser axles (still with C-clips), so I needed a brake kit compatible with that.

I considered the G-body rear disc swap options, and settled on modifying an A-body KORE3 kit with parking brake for several reasons. The LS1 rear disc swap is popular, but the downsides include shock relocation and potential contact between the caliper and frame on severely lowered cars. The C5/C6 rear rotor is shallower, so the caliper sits farther outboard. Coupled with the clocking being lower on the axle (3 or 9 o’clock), packaging is improved over the LS1 swap. The Baer rear disc conversion is nice, but the corresponding Track4 front brakes need more wheel clearance than the C5/C6 front brakes. I’m using 17” Billet Specialties Street Lite wheels (at least for now), so wheel clearance was critical in my selection. Lastly, converting to a Corvette-based brake setup allows for various rotor sizes and aftermarket rotor/caliper options for potential future upgrades.

The KORE3 kit utilized for the project is located here: http://www.kore3.com/proddetail.php?prod=10277-01

This project actually started when I got a great deal on a lightly used set of KORE3 caliper brackets and parking brake plates from a friend who put the Ford ends on his 12 bolt. I bought brand new C5 rear calipers, DBA4000 rotors, hoses and miscellaneous parts from Tobin to complete the kit.

G-bodies have the same axle housing flange as a 3rd gen F-body, so the first step is to convert the flange to a GM standard 4 bolt pattern. There is plenty of information out there on how to do this, so I’m not going to regurgitate it here. The thirdgen.org site has a descriptive set of instructions for this process: http://www.thirdgen.org/ls1reardisc

The only step I added was to have a local machine shop turn an aluminum spacer to fit between the axle tube outer diameter and the caliper bracket inner diameter for use while drilling the new housing flange pattern. This ensured the holes would be in the right spot. Overkill, but I tend to be cautious. Here’s a picture of the spacer…


Here’s the critical attribute for why the KORE3 kit is not a bolt on to a G-body. The KORE3 kits are designed for a rear end housing flange to axle flange offset of either 2.75” or 2.81”. G-body offsets tend to be less than that, but can vary based on how the carrier ends up shimmed in the rear end housing. My offsets were 2.77” and 2.68”. This does not only affect the parking brake engagement, but also affects how the caliper abutments are centered over the rotor. Here’s a picture that illustrates how the kit lines up when mounted. It’s hard to see the exact details, but you do not have a ton of tolerance in the caliper abutment to be off by 0.060” or more.


With assistance from Tobin for stress analysis, we decided to mill pockets in the caliper brackets to center the abutments over the rotor. The axle side with the short offset ended up being thinned to 0.1875” where it attached to the housing end (with Tobin’s okay). With bracket being bolted to the housing end, you essentially have a composite member at that location. Here are some pictures of the milling process and the brackets after replating. (as a side note, the brackets I got used were for the 2.81" offset design, so they are 5/16" thick... hence why the milling looks so deep. If starting with a 1/4" bracket from KORE3 for the 2.75" offset, there wouldn't have been so much material removal).




I also had the axle flanges turned down to 5.875” diameter with a 1/8” x 45* chamfer on the edge, and ARP studs added. The turning wasn’t necessary for the DBA rotors I utilized, but other rotors may not fit in the future. I also had the calipers powder coated by Mike at thepowdercoater.com (great customer service and results).

Did a final mockup of everything before painting the rear end, including routing the brake lines.


The rest of the assembly process is standard working through the KORE3 supplied instructions.




Here’s a picture of the frame rail clearance to the caliper at full bump travel:


Here’s a picture of shock clearance:


The project met all my goals: having a parking brake, requiring no shock relocation and having no interference with the stock rear frame rails at full suspension compression. I also deleted the factory combination valve to use adjustable proportioning to the rear brakes; I’ll discuss that process separately. The design of this KORE3 kit is impressive, and Tobin's knowledge and customer service were integral to completing this project.

If I had to consider doing this again, I would see if Moser could supply a custom length C-clip axle (0.070” longer) rather than go through the milling process. The key is to check axle offset before ordering parts and develop your plan accordingly. As another alternative, if you convert the 10 bolt to Ford ends, the axle offset is easily controlled and you can use a different KORE3 kit.

To clear the 17” Street Lite wheels, I ended up using a 3/16” spacer on the rear wheels, and a 5/16” spacer on the front. The rear only really needed a 1/8” spacer, but nobody makes a decent spacer (center piloting) in that thickness.

By now, you probably think I'm nuts, but that pretty much wraps it up. I have the parking brake cables routed (Lokar) but not tied in yet. I will update this once I complete the parking brake connection in a few weeks.
If the pictures quit showing up, I'll get them hosted on this site tonight and update the original post accordingly. Sorry about that...
Great info, I will reffer to this during my conversion process and will also order the Kore3 brackets needed.

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Great post. Thanks for the info, its a shame the C5 disc kit isn't more "bolt on". Mike aka thepowdercoater does great work, calipers/brackets/sway bar on my vette were coated by him