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Street Race Truck More... The Toolbox

'54 Belair - Rear Disc Brake Fix

7/31/2020

Sometimes you have to fix other people's mistakes on things when building a street rod, hot rod, rat rod, or any vehicle. The used rear axle I bought for my open drive conversion is a GM 8.5" 10 bolt positraction with 3.73 gears. Perfect for what I wanted to do. Only one issue I had was someone had already mounted Speedway Motors Weld-On Rear Disc Brake Kit with 78'-88' GM Metric Calipers in 3" Backspace. This is a very nice kit and is adaptable to multiple rear axles with a 3" diameter tube. Installation is pretty straight forward with this kit. Remove the drum brakes and backing plates and install the rotor over the axle. Hold it in place with a couple of lug nuts. Then bolt the bracket to the calipers with the brake pads installed and hold the caliper in place over the rotor and position the bracket on the rear axle tube. You can tack weld in place and check slide bolt clearance on the edge of the rotor and check the caliper is centered on the rotor. If everything looks good just finish welding the bracket to the housing. Here is where the problem comes into play with my rear axle. Whoever welded the caliper bracket on set it right on top of the axle.

With it located there, the bleeder screw was at the same level as the inlet. This made it very difficult to bleed the brakes. The preferred location would be behind or in front of the axle with the bleeder pointing up. I also had issues of wedging the tire between the caliper and lip of the body when trying to remove or install the tire. And lastly, I noticed the steel hat style rotors had a lot of run out and didn't turn very true. So a change was needed to rectify this situation.

I decided to go with a bolt-on caliper brackets for GM metric calipers since the axle flanges were still in good shape on the rear axle, part number 91031983, along with the hardware kit part number 91031983-2.

Switching to this bracket required different rotors as well. The previous weld on kit uses a 12.19-inch rotor that is 0.81" thick. The new brackets require an 11-1/4-inch diameter vented rotor 1-inch thick. The rotors are dual-drilled for both 5 on 4-1/2-inch Ford wheel bolt pattern and 5 on 4-3/4-inch Chevy. My rear axles are using the 1/2"-20 wheel studs and the rotors are drilled for 7/16-inch stock Chevy studs. So I did have to drill the Chevy bolt pattern of the rotor out to 1/2-inch to fit over the studs. With the rotors fit on to the axles and the new brackets bolted to the axle flange the calipers fit perfect and I had them in a position for easy brake bleeding and room to get my tire on and off.

This was by far much easier than welding on new brackets to reposition the caliper and it solved the issue of run-out with a 1-piece rotor.

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