What Size Brakes For My Backing Plates

[e50095]

Through the years I've picked up multiple pairs of 11" backing plates off large 8 3/4 cars. Now I'm considering using a set on a 73 Duster 8 1/4. How can I tell which size drums and shoes I need to order. I read somewhere they are not interchangeable, and the possibilities are 11x2, 11x2.5, and 11x3 shoes. I never heard of 11x3, I wonder what they came on. Anyway, does anyone know how I can identify which size backing plates I have.

Appreciate it guys...

 

[Dana67Dart]

1. look for part numbers on the backing plates
2. compare the depth of where the housing flange mates to back of each backing plate. (deeper is wider show)
3. buy a set of each width shoes and test fit with known drums (typically the machined surface of the drum will be a bit wider than the shoe it is designed for)

 

[e50095]

So on our sister site forbbodies a cat responded to an older similar question that the 11x3 never came on the rear, it was a front brake for circa 1970 B bodies, and that the vast majority of 11" rear plates took the 2.5" shoe. Given that, I'll follow Dana67Dart's suggestion to put the plates on with a 11x2.5 drum I have and test fit some shoes.

 

[Dana67Dart]

plates on with a 11x2.5 drum I have and test fit some shoes.

note that you could probably use 11 x 2 shoes in an 11 x 2.5 drum / backing plate combo without issue ( just less braking ability)

 

[cudamark]

11 X 3" rear brakes came on many C-bodies, especially wagons, but, also police/taxi and tow package cars. The backing plates must match the shoes they're supposed to have or the shoe will be cocked, and initially only wear on either the inner or outer edge depending on the mismatch. Take a pair of backing plates with you to the parts store and try each option to see how they fit. the shoe surface must be 90 degrees from the backing plate so the shoe contacts the drum evenly. Then you can get the correct hardware kit and drums to match. If you just have bare backing plates, you may have trouble getting all the hardware for some applications. If you have a pair for the 11 X 2 1/2" brakes. I'd use those. They're the easiest to get parts for. The 11 X 2" are probably the next easiest as they use those on the 1/2 ton pickups quite a bit. The 11 X 3" might be overkill for some applications, but, will work on lighter cars if you use an adjustable proportioning valve. The Imperial had an 11 X 3 rear brake too, but, also a unique backing plate that was different than the standard C-body one. It might work if you can find all the Imperial parts to go with it. They had a different bolt pattern wheel, so, drums might be an issue.

 

[e50095]

thanks brother, I think it best to avoid the 11x3 altogether.

 

[72bluNblu]

The backing plate offset is different for all 3 versions of the 11” drum brakes. Backing plates, shoes and drums can not be interchanged between the different widths.

To measure the backing plate offset you want to set the backing plate flat on a bench. You'll need to hang the bottom 1/3 of the backing plate off the edge of the bench so that the mounts for the e-brake cables don't keep the backing plate from sitting flat. It isn't much, but they do change the measurement. Then, measure from the bench to the top of the backing plate through the center hole (for the axle) in the backing plate.

If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates. Those I know for sure, as I've measured them. I don't have a set of 11x3" backing plates, but the math would say you'd get ~2" measuring them as I described above. This is because half of the additional width is on the inside of the axle flange and the other half is on the outside to keep the axle flange in the same spot, as there was only one axle offset for BBP 8 3/4 axles.

note that you could probably use 11 x 2 shoes in an 11 x 2.5 drum / backing plate combo without issue ( just less braking ability)

Why? The 11x2.5” shoes are easy to find, 11x2’s are harder to come by. And the backing plates and drums have to match otherwise the drums hit the backing plates or leave a gap, neither of which are serviceable.