7 1/4 To 8 3/4 drum brakes

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Vitamin c

Vitamin c

Vitamin c
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Are the 7 1/4 and 8 3/4 SBP drums, backing plates, shoes, the same? I am stripping down a 72 6 cyl Demon with a 7 1/4, and will be installing an 8 3/4 SBP in its place. Before I pitch the whole 7 1/4 setup, should I save the backing plates etc to help assemble the new 8 3/4. I have an 8 3/4 empty housing but don't have the axle end/brake hardware yet.
 
I don't think they will line up. I would still save them some one would need them.
This might help you
 

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well I've seen that axle ID sheet, but it doesn't show the 7 1/4 (i'm assuming that's what's in the demon). I didn't think to look at the bolt pattern when I had the drums off.
 
Just went through this on Friday. Nothing from your 7 1/4 will work on the 8 3/4...NOTHING. Even the ubolts and shock plates are different. The only thing you can save would be the leaf springs.
 
if they are 10" brakes the drums are the same between the A body 7.25 and 8.75 assuming they are both small bolt pattern. The backing plates have different offsets and can not be interchanged.
Personally I wouldnt bother with the small bolt pattern stuff unless I had to.
Buy a new set of large bolt pattern axles set up for the standard MOPAR brake offset and use any large bolt pattern rear brake set-up. 7.25 to dana, it all swaps.
 
Actually, if you're working with SBP on both, and your 7-1/4" has 10" brakes, everything except the backing plate will 100% interchange. Backing plates MIGHT work with one hole drilled for the lower centre stud.
With 9" brakes, only one way to be sure...
 
dusterglenn said:
Actually, if you're working with SBP on both, and your 7-1/4" has 10" brakes, everything except the backing plate will 100% interchange. Backing plates MIGHT work with one hole drilled for the lower centre stud.
With 9" brakes, only one way to be sure...
Click to expand...


Backing plates will NOT work. The offset is different btwn the A body 7.25 and 8.75. It has nothing to do with the number of mounting holes.
 
Jimacuda said:
if they are 10" brakes the drums are the same between the A body 7.25 and 8.75 assuming they are both small bolt pattern. The backing plates have different offsets and can not be interchanged.
Personally I wouldnt bother with the small bolt pattern stuff unless I had to.
Buy a new set of large bolt pattern axles set up for the standard MOPAR brake offset and use any large bolt pattern rear brake set-up. 7.25 to dana, it all swaps.
Click to expand...
Yes, the drums are 10'' and are the "bell" style. that's what got me thinking. I have done a couple small to large BP 8 3/4 conversions and know what's involved. I want to keep this car with the small pattern and use the stock wheel.

dusterglenn said:
Actually, if you're working with SBP on both, and your 7-1/4" has 10" brakes, everything except the backing plate will 100% interchange. Backing plates MIGHT work with one hole drilled for the lower centre stud.
With 9" brakes, only one way to be sure...
Click to expand...
If the drum is the same, wouldn't that mean the backing plate offset is the same???

Jimacuda said:
Backing plates will NOT work. The offset is different btwn the A body 7.25 and 8.75. It has nothing to do with the number of mounting holes.
Click to expand...
As I asked above, if the drum is the same, wouldn't the offset of the backing plate be the same. I never did think to look at the mounting holes.
 
Offset is not the same, believe me. I have compared the 7.25 and 8.75 A body backing plates side by side and the difference is very noticeable to the naked eye. Both the backing plate and axle offset are different between the A body 7.25 and 8.75.
Why Chrysler in their infinite wisdom decided to make the A body 8.75 brakes unique unto itself I have no clue, but they did.
 
I just wanted to know this before I pitch the axle. I think I have a pair of A-body SBP backing plates somewhere "I hope" It would be a shame to junk them if they would work on the 8 3/4
 
Hello, if the drums are actually 10" small bolt pattern, they will work on original 83/4 a body. I have had both. Switched the drums around. The brake shoes are the same as well. 10" x 13/4". The backing plates are different. Biggest difference being the bolt pattern. 71/4 has a 4 bolt pattern, and 83/4 has a 5 bolt pattern. Offset is the same.
 
1972 dodge said:
Hello, if the drums are actually 10" small bolt pattern, they will work on original 83/4 a body. I have had both. Switched the drums around. The brake shoes are the same as well. 10" x 13/4". The backing plates are different. Biggest difference being the bolt pattern. 71/4 has a 4 bolt pattern, and 83/4 has a 5 bolt pattern. Offset is the same.
Click to expand...

The bolt pattern on the backing plate doesn't matter at all, the offset is the problem. BBP 7.25" backing plates only have 4 bolts, they work fine on BBP 8 3/4's if you drill the 5th hole and are identical otherwise.

The SBP 7.25 and 8 3/4 drums and hardware are the same, the backing plates ARE NOT. The offset on the axle flange is different between the two axles, and therefore the offset on the backing plates is different. The only backing plates that can be used on SBP 8 3/4 axles are SBP 8 3/4 backing plates. Same for SBP 7.25 axles, they can only use backing plates from a SBP 7.25.
 
All my brake parts on my BBP 7 1/4 switched over to my 65 B body 8 3/4 (except brake lines). That's all I know. Might want to hold on to your old stuff and see. :dontknow:
 
Lelo Dart said:
All my brake parts on my BBP 7 1/4 switched over to my 65 B body 8 3/4 (except brake lines). That's all I know. Might want to hold on to your old stuff and see. :dontknow:
Click to expand...

That only works for BBP. BBP axles all have the same axle flange offset. You can swap brakes between the BBP 7.25, 8 3/4, even Dana 60's.

The SBP 8 3/4 axles have an axle flange offset that is unique to SBP 8 3/4 axles. You can't swap the backing plates with anything, even SBP 7.25" axles.

It's been posted here many, many times by many different people. SBP 8 3/4 axles have a flange offset that is unique to SBP 8 3/4 axles ONLY. The backing plates don't interchange with anything.
 
Are the parking brake cable bracket and strut interchangeable between the 7 1/4" 10" drums and the 8 3/4" 10" drums???
 
Well it looks like I can pull the shoes, hardware, Ebrake cables, drums, and pitch the rest. The oddball 8 3/4 axles require a special backing plate, and I think I have a se, ill have to dig through some parts. .
 
Is there any way to identify sbp 8 3/4 backing plate by just looking at it? Like scrounging at a swap meet? How would you know?
 
d fark said:
Is there any way to identify sbp 8 3/4 backing plate by just looking at it? Like scrounging at a swap meet? How would you know?
Click to expand...

The depth of the backing plate is the only way to know. You have to measure the depth of the recess in the backing plate where it bolts to the axle housing. I don't remember the depth measurements of the SBP brakes compared to the BBP brakes, but I know I've posted the measurements for all the BBP backing plates on here before.
 
d fark said:
Is there any way to identify sbp 8 3/4 backing plate by just looking at it? Like scrounging at a swap meet? How would you know?
Click to expand...

I'll see if I can take a pic tonight but the BBP and SBP 8 3/4 plates do look slightly different.
 
72bluNblu said:
The bolt pattern on the backing plate doesn't matter at all, the offset is the problem. BBP 7.25" backing plates only have 4 bolts, they work fine on BBP 8 3/4's if you drill the 5th hole and are identical otherwise.

The SBP 7.25 and 8 3/4 drums and hardware are the same, the backing plates ARE NOT. The offset on the axle flange is different between the two axles, and therefore the offset on the backing plates is different. The only backing plates that can be used on SBP 8 3/4 axles are SBP 8 3/4 backing plates. Same for SBP 7.25 axles, they can only use backing plates from a SBP 7.25.
Click to expand...
Hi 72bluNblu,
I bought a 8 3/4 rear with the smp axles. Also, it can with matching backing plates. I have a 7 1/4 rear now with 10 inch drums that have 1.75 inch wide shoes, sbp.
Is it safe to say I just need to use the backing plate that came with the rear, and then my 10 inch drums, shoes and hardware will just bolt all up? But then if I wanted the 2.5 diameter shoes, its just a matter of buying new drums and shoes and they would bolt right in?
I see others talking about the drums that flare out but mine are not like that, and just flat looking.
Thanks!
 
ChuckDock said:
Hi 72bluNblu,
I bought a 8 3/4 rear with the smp axles. Also, it can with matching backing plates. I have a 7 1/4 rear now with 10 inch drums that have 1.75 inch wide shoes, sbp.
Is it safe to say I just need to use the backing plate that came with the rear, and then my 10 inch drums, shoes and hardware will just bolt all up? But then if I wanted the 2.5 diameter shoes, its just a matter of buying new drums and shoes and they would bolt right in?
I see others talking about the drums that flare out but mine are not like that, and just flat looking.
Thanks!
Click to expand...

If you use the 10” SBP backing plates that came with the 8 3/4 you should be able to use all the other brake hardware from the 7.25 10” SBP brakes- drums, shoes, self adjusters etc. The 10” drums with the flare or “bell” was just a design thing, they made them both ways and other than the extra flange the dimensions are the same.

10x2.5” rear drums are all BBP. The axle flange offset is different from the SBP axle flange offset. So, if you want to use 10x2.5” BBP rear brakes later you will need new axles, new backing plates, drums and shoes. The hardware, like the spreader bar and parking brake levers can be reused. But that’s it.
 
72bluNblu said:
If you use the 10” SBP backing plates that came with the 8 3/4 you should be able to use all the other brake hardware from the 7.25 10” SBP brakes- drums, shoes, self adjusters etc. The 10” drums with the flare or “bell” was just a design thing, they made them both ways and other than the extra flange the dimensions are the same.

10x2.5” rear drums are all BBP. The axle flange offset is different from the SBP axle flange offset. So, if you want to use 10x2.5” BBP rear brakes later you will need new axles, new backing plates, drums and shoes. The hardware, like the spreader bar and parking brake levers can be reused. But that’s it.
Click to expand...
Awesome, good info for sure! Tomorrow I"ll start to work on it.
Thanks!
 
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