Backing plate offset

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Der Sebi

Der Sebi

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Hi all,

I know there are already several threads regarding backing plate offset. But these just added some confusion:???:
I am currently building a 8,75" rear end for my ´72 Duster. The housing is an early B unit which has been narrowed to A-Body factory specs. I bought SBP axles from Moser (also factory specs, I guess 27,***" length...).
My question: can I use the backing plates which come from the Dusters original 7,25" rear end? Or can I use the units which came with the B-Body rear end?

Thanks in advance and greetings from Germany :)
Sebastian
 
It all boils down to how bad you want to keep the SBP wheels. The 8 3/4" rear has 5 studs in the housing end and the 7 1/4" has 4. You can either drill the 5th hole in the 7 1/4" backing plates or knock that stud out of the housing end. Assuming the 7 1/4 " rear had 10 inch brakes they would be 10" X 1 7/8" and the supply of replacement parts are drying up. They haven't been produced on a new car since 1972. The price for drums is ridiculous when you can find them and the shoes aren't much better. The B body brakes are 10" X 2 1/2" and since these were produced into the late 80s the part availability is much better (dirt cheap). To use them you will need to redrill your axles to 5 on 4 1/2" bolt circle (if they aren't already) and get a set of 1/2" diameter studs, nuts, and BBP wheels. You pays your money and you takes your chances...
 
Der Sebi said:
Hi all,

I know there are already several threads regarding backing plate offset. But these just added some confusion:???:
I am currently building a 8,75" rear end for my ´72 Duster. The housing is an early B unit which has been narrowed to A-Body factory specs. I bought SBP axles from Moser (also factory specs, I guess 27,***" length...).
My question: can I use the backing plates which come from the Dusters original 7,25" rear end? Or can I use the units which came with the B-Body rear end?

Thanks in advance and greetings from Germany :)
Sebastian
Click to expand...

Curious as to why you bought small bolt pattern spec axles?
That limits you to A body 8.75 10" brakes only, using factory parts.
A body 8.75 backing plates are unique, only used on 72 and older A body 8.75's.
10" small bolt pattern rear drums are interchangeable between the 7.25 and 8.75 but not the backing plates. Different offsets.
 
Here is a thread that gives the real world numbers on the backing plate measurements. "35/64 or 0.5468 of an inch" http://www.forabodiesonly.com/mopar/showthread.php?t=284979

The second issue is going to be the hub is way too small "if it was manufactured to the OEM size" to use with the modern 10" lbp drums. One member has a special ring that he made or had made to correct this problem.

Since the backing plate difference is 0.5468" and the lbp drum difference is .75", that is something that would have to be carefully looked at.
 
grumpuscreature said:
It all boils down to how bad you want to keep the SBP wheels. The 8 3/4" rear has 5 studs in the housing end and the 7 1/4" has 4. You can either drill the 5th hole in the 7 1/4" backing plates or knock that stud out of the housing end. Assuming the 7 1/4 " rear had 10 inch brakes they would be 10" X 1 7/8" and the supply of replacement parts are drying up. They haven't been produced on a new car since 1972. The price for drums is ridiculous when you can find them and the shoes aren't much better. The B body brakes are 10" X 2 1/2" and since these were produced into the late 80s the part availability is much better (dirt cheap). To use them you will need to redrill your axles to 5 on 4 1/2" bolt circle (if they aren't already) and get a set of 1/2" diameter studs, nuts, and BBP wheels. You pays your money and you takes your chances...
Click to expand...

Early 7.25 backing plates will not fit the 8.75 ends, even if you knock out the stud...different size pocket. The 73-up 7.25 plates are interchangeable though.
You could simply drill the SBP into the later drums, but the problem you're going to have is the register on the axle flange. The Drum would only be centered by the studs and that's not a good plan at all.
 
whitepunkonnitro said:
Early 7.25 backing plates will not fit the 8.75 ends, even if you knock out the stud...different size pocket. The 73-up 7.25 plates are interchangeable though. You could simply drill the SBP into the later drums, but the problem you're going to have is the register on the axle flange. The Drum would only be centered by the studs and that's not a good plan at all.
Click to expand...

Thank you for confirming what I was recalling about the backing plates. I was just going to look up the correct information on that and post it to this thread.
 
First of all thanks to all of you for your info!:)

Regarding my bolt pattern choice: I just wanted to avoid buying new wheels and wasn´t really aware of the problems I´m having right now...

So what would be the best option in my case?
I guess locating the correct SBP-8,75"-backing plates is close to impossible...

Should I get the axles redrilled to BBP and take 10" drums off any other rear end?
If so, could I use the 7,25" backing plates or the ones that came with the Early B rear end then?
 
Your axle offset choice is what can limit you to using only the 72 and older A body 8.75" rear end 10"x1.75" brake backing plates. The distance the axle extends from the flange it bolts to is what determines the offset you need for the backing plates.
Just redrilling the axles will not solve that problem. The center register will be too small for any mopar brake drums except the A body small bolt pattern drums.

Just to clarify, what exactly did you get for axles? You stated "I bought SBP axles from Moser (also factory specs, I guess 27,***" length...)."
What is the center register diameter? Thats very important.
Any other specs listed for them?
 
from the ad:

"Part # A30-CST-A4- A-Body, 8 3/4" Rear, Stock A-Body Drum Brake -- 30 Spline"

With that description I'm going to say you'll have to use the A body 8-3/4 backing plates.



So to answer your original question:

"My question: can I use the backing plates which come from the Dusters original 7,25" rear end? Or can I use the units which came with the B-Body rear end?"

You can not use either set of backing plates. If the original drums from the 7.25" are 10" you can use those but you'll need the 8.75 rear drum backing plates.
I know, it all gets very confusing.
 
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