11" Backing Plate Differences

[RogerRamRod]

I have 11 x 2-1/2 drums. I'm under the impression that there are different Backing Plates for 2" -vs- 2-1/2" shoes/drums. Do you know if that's correct?

Somebody in the past put a spacer under the drum. If I remove the spacer, the drum scrapes against the outer lip of the backing plate.

I'm not really sure what to measure. I currently have 11" backing plates, so I'm assuming there is a different "Depth" to different backing plates.

Does anybody know if there is a part number usually located in a usual spot under all the crud where I should be looking? Or do I need to remove them to lay them on a flat surface to get a measurement?

Thanks in advance for any info you can provide.

I posted this on B-Bodies as well since this is a B-Body rear I'm using.

 

[Dana67Dart]

Typically the backing plate and drum and axle all have to agree.

If the backing plates are for an OTHER than A body then they are in agreement with OTHER than A body axles.

Then they have to be in agreement with the width of the drum.

A 2" depth backing plate will rub a 2.5 drum, a 2.5 depth backing plat will not rub a 2" drum but the shoes will not be on the correct part of the drum and might not be making contact on the inside nearest the backing plate.

What I would do is measure from the outer face of the axle flange to the lip of the backing plate and then look at drums for the other widths available.

There might be 2, 2.25, 2.5, 2.75, 3"
This all assumes that you have stock mismatched components.

Good luck.

It's not impossible just time consuming.

 

[72bluNblu]

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.

 

[RogerRamRod]

Thank You for that bit of info, thats exactly what I was looking for. The spacer under the drum doesn't bother me so much, It allows the 2.5 shoe/drum to work with the 2" backing plate, except the wheels I'm interested in have about .5" different offset, pushing the outside of the wheel/tire closer to the quarter, where I'm already pretty close. It only buys me 1/4" per side though because thats about what those spacers are, and makes perfect sense after reading- "If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates."

Maybe I'll just stay with the wheels I've been using until I decide if I'm going to utilize a modded Exploder 8.8 which is about an inch narrower than my current rear with said spacers. And that would get me my 1/2" needed per side.

 

[MoparMike1974]

Basically they widened the brakes by moving the backing plate back. You must match the plate with the drums / shoes you are using.