11" Backing Plate Differences
The 11" drums come in three variations as far as I know, 11x2", 11x2.5" and 11x3". The axle flange stand off is the same for all BBP axles, so, the difference in the offset is all in the depth of the backing plate. I don't know of any numbers stamped on the pates to identify them by, I just always measured them.
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. If you just lay the whole plate on the bench the little bumps for the e-brake mounts cause it to not sit 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.
All of that said, if you have 11x2.5" drums that rub on the backing plates it's a really good bet that what you have is 11x2" backing plates. So, you either need to get new backing plates, or new drums and shoes. It would be easier to get new drums and shoes, nothing wrong with the 11x2" brakes.
The 11" drums come in three variations as far as I know, 11x2", 11x2.5" and 11x3". The axle flange stand off is the same for all BBP axles, so, the difference in the offset is all in the depth of the backing plate. I don't know of any numbers stamped on the pates to identify them by, I just always measured them.
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. If you just lay the whole plate on the bench the little bumps for the e-brake mounts cause it to not sit 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.
All of that said, if you have 11x2.5" drums that rub on the backing plates it's a really good bet that what you have is 11x2" backing plates. So, you either need to get new backing plates, or new drums and shoes. It would be easier to get new drums and shoes, nothing wrong with the 11x2" brakes.