anyone know how to convert over to 12 inch backing plates and rear drums?

[cutt]

???

 

[Red Regal T]

If you're talking about older GM backing plates, they bolt on to 8.5's flange but you have to cut off the upper part of the flange because it hit's the backing plate and prevents it from being installed. Brake springs attach and brake shoes butt up against the top stud on backing plate of larger brakes while the stock 8.5 top stud is attached to rear flange.

 

[turbofish38]

I dont know of a 12" drum with a 4.75 bolt pattern. All GM 12" are 5X5". Now you can put 11" drums but I wouldn't recommend it with a stock 8.5 TR axle. If you cut the flange like John says you remove 50% of the backing plates clamping area. And the little 8mm bolts that are there are IMO to small for anything and there isn't enough meat on the flange to put anything bigger. I run 11" drums on the back of my GN but I am also running a different axle set up so they fit properly. Good luck if you do manage to accomplish this with a stock axle as you'll be able to powerbrake your car past 20# of boost on the line without the wheels breaking loose.

 

[Red Regal T]

I forgot what size it was but I was describing the installation of 11" brakes found on 73 thru 77 Cutlasses and other mid-size GMs. You have to cut the top part of the flange off our 8.5 housings to install these 11" backing plate. The four bolts that hold on the backing plate to the flange is the same system that fastened many big block A-body musclecars of the 60s and 70s.

 

[cutt]

oem drum size

what is the original drum size?

 

[v6turbojunkie]

Originally posted by turbofish38
I dont know of a 12" drum with a 4.75 bolt pattern. All GM 12" are 5X5". Now you can put 11" drums but I wouldn't recommend it with a stock 8.5 TR axle. If you cut the flange like John says you remove 50% of the backing plates clamping area. And the little 8mm bolts that are there are IMO to small for anything and there isn't enough meat on the flange to put anything bigger. I run 11" drums on the back of my GN but I am also running a different axle set up so they fit properly. Good luck if you do manage to accomplish this with a stock axle as you'll be able to powerbrake your car past 20# of boost on the line without the wheels breaking loose.

I have the same setup. Sent you a PM.

 

[v6turbojunkie]

Re: oem drum size

Originally posted by cutt
what is the original drum size?

9" if memory serves.