Hello guys, still in the process of re-doing the rear brakes on my 1973 roadrunner. I have attached some pictures of measurements because I keep running into issues with the assembly of the drums. From the research I've done, I believe I'm supposed to have 11in x 2 1/2in brakes in the rear. Measuring the inner diameter of the old drum, I get somewhere around 11 inches and the width of the surface where the shoes ride on is just under 3 inches. But here is where I run into an issue; I measured the width of the shoe that I took off and it is 2 inches. Shouldn't this read 2 1/2in? I ordered 2 1/2in shoes without even measuring the old ones because I assumed that is what was on there but after putting them onto the old backing plate with all the hardware, it's not a proper fit. I read that some older Mopar trucks use 2in brakes. Did the previous owner maybe put a backing plate and shoes that came off a truck with a drum that belongs on this car? Any help is appreciated, thanks everyone.
Drum and Shoe Size
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11x2" brakes came on a lot more than just trucks, it's entirely possible your car just came with 11x2's. 11x2.5" was the most common, but there were 11x2" and 11x3" that were used as well. You're going to need to measure the offset on the backing plate flange.
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.
It's hard to see in this picture, but the edge of the bench is on the left side and the bottom of the plate is hanging off so the bumps for the parking brake cables don't keep the plate from sitting flat.
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.
It's hard to see in this picture, but the edge of the bench is on the left side and the bottom of the plate is hanging off so the bumps for the parking brake cables don't keep the plate from sitting flat.
I appreciate that. I just went and measured and I got 1.75" which means my shoes should be 11x2.5". I tried putting new 11x2.5" shoes on that backing plate a couple days ago but the way the top of shoes encompass that round elevated bit where the springs and self adjuster cable get connected to doesn't seem right. The shoes encompassed that piece too high up and didn't allow enough room for both springs to sit. I'll give it another shot tonight and let you know how it goes, thanks!
I have attached a couple different photo angles to show how the new 2 1/2in shoes sit in relation to the spring and cable holder. They are sitting a bit too high up and both springs won't fit under the groove. Should I find a thinner plate that sits under the self adjuster cable and springs? That'll probably cause both to fit but I want to make sure that the shoes are okay to sit like that. Thanks
It doesn't look like you've got everything assembled, so I wouldn't be too worried about it. The shoes have to be sitting correctly on the little platforms the shoes ride on, and if they're not they won't sit properly. The way you have them sitting on there the wheel cylinders could be holding them up. If the measurement on the backing plate offset you took is correct then those should be 11x2.5" backing plates, so fully assembled the shoes should sit down.
These are from when I had 11x2.5" brakes on the back of my Duster
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