Does anyone know the size of the backing plate small bolts used in the 7.5 and 8.5 10 bolts? Wondering if I should try to find these bolts or just drill them out and use a 1/4” nut and bolt.
Brake backing plate change
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Drill them out if they are broke. I’ve tapped them for standard 1/4” bolts and used a nut and bolt. Both will work fine as long you get the backing plate in straight.
I got the broken pieces out already. The housing is buried under the back of the other car so I can’t check it. But if I can tap it for 1/4” I’ll do that. I figured it was so close in metric size that I wouldn’t be able to tap it.
Best info I can find on these is GM p/n 563390. M6.3 x 1 x 18mm long. Hope that helps some. Weird azz bolt sizes for sure.
Have to wonder why in the heck that they did that at all. Instead of using pre-existing 9.5 inch diameter rear backing plates, and related hardware that had existed for over a decade, by that point.
Since it's GM, it was because soemone was convinced it would save money. GM went back to the same trapezoid pattern for the Camaro disc brake plates in the 1998+ Camaros. I lined one up, and the upper holes in the LS1 plates lined up perfectly with the factory clip-on cylinder backing plate holes on the axle tabs. Like pilot holes. 
Well the new designs are much better than original. I changed the backing plates on my 7.5 rear in the car when I got it. The new backing plates are keyhole shaped Along with the wheel cylinder to prevent rotation and the new cylinder has wings to hit the housing in the event of rotation. The old design was a mess. I believe gm had an anti rotation device for the wheel cylinder as a service bulletin.
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