swapping 8.75 for 7.25, brake drum problem

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screeminDemon

screeminDemon

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just about completed the swap on my old slant 6 demon, which is now a 360 Demon with 8 3/4 rear... but the brake drum issue is stumping me. I've ordered from Summit... drums were correct 10" but too deep (3 3/4"). Same thing with drums from autozone. The drum hits the rear end housing backing plate before all the studs are fully thru the holes.

Note: the original drums on this 8 3/4 housing were small bolt pattern, and they were pretty much shot anyways.

Any suggestions? Do I need goto a junkyard? Do I need to get different housing backing plates? Thanks for all your help!
 
Yes, you have to swap the backing plates. Any Aspen / Volare / Diplomat 10 X 2 1/2 rear brake backing plate will work.
 
thanks! thats what I needed to know. Now I'll hit the phones and see what the local junkyards have left around here...
 
well that turned out rather disappointing... I called all the local junkyards and it seems there are very few 70's and 80's mopars left out there. I only found one junkard that had a couple mopars, not a-body's, and they dont want rear end housings taken apart with axles out and everything, just to sell the backing plates.

Any other suggestions?
 
Im confused.. the 8 3/4 had drums and backing plates on it when you got it but the drums were shot right? it was all small bolt.. did you stay with small bolt or go big bolt? regardless, I was under the assumption that 8 3/4 backing plates for 10" brakes were all the same? I was also under the assumption that you can no longer get rear drums at the auto parts stores anymore.. Someone please correct me if I am wrong.. when I went to get new drums for my Dart (8 3/4 with 10" brakes.. small bolt pattern) drums were not available.. I had to do the swap meet thing for a while to find a decent set..
 
SpeedracerX said:
Im confused.. the 8 3/4 had drums and backing plates on it when you got it but the drums were shot right? it was all small bolt.. did you stay with small bolt or go big bolt? regardless, I was under the assumption that 8 3/4 backing plates for 10" brakes were all the same? I was also under the assumption that you can no longer get rear drums at the auto parts stores anymore.. Someone please correct me if I am wrong.. when I went to get new drums for my Dart (8 3/4 with 10" brakes.. small bolt pattern) drums were not available.. I had to do the swap meet thing for a while to find a decent set..
Click to expand...

I went to the big bolt pattern. The drums that came with the 8 3/4 housing had small bolt pattern and they were about 1 inch shallower than the drum sold to me by summit. The deeper drum bottoms out on the backing plate before the wheel studs are all the way thru the holes.

I just went out and bought 5/16" spacers from autozone, which is probably a bad thing to do, but it will allow me to seat those drums and drive... until I can find another solution - like hitting some swap meets I guess
 
You might have to drill a set of small bolt pattern drums to fit large bolt pattern. I think you can buy new drums from napa for the small bolt pattern and have them drilled to fit.
 
8 3/4 backing plates are different from small bolt pattern to large bolt pattern and the drums do not interchange. You could drill some small bolt drums, change the backing plates or there is a drum that will fix you up but they are very hard to find. I sold my last set at Chrysler Carlisle. Here is a current e-bay auction for them by someone http://www.ebay.com/itm/220800761284?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 I do have a set of nice used backing plates if you decide to go that way and need them PM me
 
abodyfan said:
You might have to drill a set of small bolt pattern drums to fit large bolt pattern. I think you can buy new drums from napa for the small bolt pattern and have them drilled to fit.
Click to expand...

Looking at my old ones it appears the center (hub) hole on the drum would have to be bout 1" diameter bigger too. That seems like alot of cutting/holes in the attaching area of the drum, but I have thought about talkin to my machine shop about it. I may give this idea some more thought... Thankyou!
 
Just list in the Wanted section, I know many people on this forum have the backing plates laying around.
 
I would try ROCKAUTO, I got two from them on closeout for like $24 apiece.

AUTOSPECIALTY / KELSEY-HAYES Part # AD8307
Front; Except Front Disc/Rear Drum; 10" X 1 3/4" or 10" X 2 1/4" 8 CYL [Wholesaler Closeout - Private Label Pkg. - 30 Day Warranty] (Only 1 Remaining)
 
When I built my 8.75 I did the following.
Bought the large bolt pattern axles from Dr. Diff.
I used the backing plates, and the drums that came off the large bolt pattern 7 1/4 that came out of my duster.
It bolts right on.
 
Which axles did you use to go big bolt pattern?

SBP axles also have a different length (offset) to use the SBP 10x1.75" rear brakes.

10x2.5" backing plates will not interchange with 10x1.75" backing plates, as you've found out the offset is different. Further, if you had your SBP axles redrilled for BBP, you have to do the same with the brakes, because 10x2.5" brakes will not fit (even with the correct backing plates) because of the axle length.

So, if you want to go to BBP 10x2.5" brakes, you need to have:

1. The correct length BBP axles (not re-drilled SBP axles)
2. 10x2.5" rear brakes, with 10x2.5" backing plates, drums etc.

10x1.75" SBP brakes only work with SBP axles and backing plates. If you redrill a set of SBP axles to BBP, you have to redrill your 10x1.75" drums too. And, some aftermarket BBP axles are set up to use the stock 10x1.75" brakes with re-drilled drums. So, make sure your axles are the right length (Dr. Diff sells the correct lengths). And of course the same is true in reverse, if your axles were set up for 10x2.5" BBP brakes, you can't use the 10x1.75" brakes, even if you have the drums redrilled to BBP, because the axle length will be wrong.

As far as the BBP 10x2.5" brakes, they're pretty much all the same. They were used up until 1989 on Diplomats and 5th avenue's, so you don't have to find a 70's car. On early 80's vehicles you can find them on just about anything, including small trucks as the lighter duty trucks had the same bolt pattern. And any BBP set up should work, whether its attached to a 7.25, 8.25, 8.75, 9.25 or Dana rear axle. Some of the 8.25" rears only used 4 studs, so you may have to drill a hole in the backing plate for the 5th stud.
 
sireland67 said:
When I built my 8.75 I did the following.
Bought the large bolt pattern axles from Dr. Diff.
I used the backing plates, and the drums that came off the large bolt pattern 7 1/4 that came out of my duster.
It bolts right on.
Click to expand...

I did the same thing, worked great.
 
72bluNblu said:
Which axles did you use to go big bolt pattern?

SBP axles also have a different length (offset) to use the SBP 10x1.75" rear brakes.

10x2.5" backing plates will not interchange with 10x1.75" backing plates, as you've found out the offset is different. Further, if you had your SBP axles redrilled for BBP, you have to do the same with the brakes, because 10x2.5" brakes will not fit (even with the correct backing plates) because of the axle length.

So, if you want to go to BBP 10x2.5" brakes, you need to have:

1. The correct length BBP axles (not re-drilled SBP axles)
2. 10x2.5" rear brakes, with 10x2.5" backing plates, drums etc.

10x1.75" SBP brakes only work with SBP axles and backing plates. If you redrill a set of SBP axles to BBP, you have to redrill your 10x1.75" drums too. And, some aftermarket BBP axles are set up to use the stock 10x1.75" brakes with re-drilled drums. So, make sure your axles are the right length (Dr. Diff sells the correct lengths).
QUOTE]

Thanks for all the terrific info! I did get my axles from Dr. Diff. It was a couple months ago. I remember I told him I was switching from 7 1/4 sbp to 8 3/4 bbp but we didnt talk about the brakes... So I hope I got the right length for what I'm doing. I decided the best way to do this is to switch to the 10" with 2.5" linings. It means finding the other backing plates you mentioned, and other matching brake parts. It's kind of a pain, but it will give me the best braking power in the end.
Click to expand...
 
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