Question after converting a 7-1/4 to 8-3/4

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19 Again..

19 Again..

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I have a question. I have a 73 Duster with a 7-1/4 in it currently, and I have an 8-3/4 I'm going to put in it over the winter. The bolt pattern for the 7-1/4 is 5 x 4.5 (LBP) and the brakes are 10 x 2.5 too. Can I change out the current brakes & hardware to the new rearend using the same backing plates? I just replaced everything last year on the 7-1/4. Second question is will I gain any width and extra back spacing when I swap out the 7-1/4 for the 8-3/4? Thanks "StrokerScamp" for suggesting starting my own thread....
"19"
 
Does the 8 3/4 have 10 X 2 1/2" brakes and a 4 1/2" bolt pattern too?
 
Yes it has 10-1/2 drums, but you made me go out and measure the studs on the 8-3/4.. If I read the tape measure right it appears the studs are at a 4" bolt pattern. In which case I'll have to have the axles re-drilled for a 4-1/2 pattern which is on the car right now with the 7-1/4. Something of which I had forgotten. Just one more thing to add to the list. Glad I looked before I got into it.. Looking at the backing plate on the 7-1/4 it looks Identical to the 8-3/4.. But the floor is pretty cold and I'm not as young as I used to be so I didn't slide underneath the car to verify the backing plate bolts. LOL!!!
 
.........If u buy bb pattern axles for ur 8.75 u can use all ur 7.25 brake assamblies including ur backing plates, the 5th hole is in the 7.25 backing plate on the bottom.......kim....
 
19 Again.. said:
Yes it has 10-1/2 drums, but you made me go out and measure the studs on the 8-3/4.. If I read the tape measure right it appears the studs are at a 4" bolt pattern. In which case I'll have to have the axles re-drilled for a 4-1/2 pattern which is on the car right now with the 7-1/4. Something of which I had forgotten. Just one more thing to add to the list. Glad I looked before I got into it.. Looking at the backing plate on the 7-1/4 it looks Identical to the 8-3/4.. But the floor is pretty cold and I'm not as young as I used to be so I didn't slide underneath the car to verify the backing plate bolts. LOL!!!
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If it's a stock A-body 8 3/4, it would have 10 X 1 3/4" brakes and the small bolt pattern. Those backing plates won't work with 2 1/2" shoes so you'll have to use the correct ones that match the 2 1/2" shoes. I'm not sure if you can use those ones off the 7 1/4 or not. It seems to me I remember something about the distance from the end of the housing to the end of the axle shaft was different on those two applications. Maybe someone that has acually done this particular swap will chime in here to verify or tell me I'm full of it.
 
Ok, resurrecting a little here, but interested...I just bought an 8 3/4" housing from eBay with axles and backing plates and was wondering about the same thing. The brakes on the 7 1/4" currently out back has low mileage piece brake parts and I'd like to know if I could swap all those pieces over to the "new" one. From what Kim is saying, I should be able to, right?
 
txstang84 said:
Ok, resurrecting a little here, but interested...I just bought an 8 3/4" housing from eBay with axles and backing plates and was wondering about the same thing. The brakes on the 7 1/4" currently out back has low mileage piece brake parts and I'd like to know if I could swap all those pieces over to the "new" one. From what Kim is saying, I should be able to, right?
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..........As long as u have big bolt pattern 8.75 axles, not redrilled 1s.....ur small bolt backing plates r not the same..even if u drill the axles to big bolt...big bolt needs all big bolt components....kim.....
 
oldkimmer said:
..........As long as u have big bolt pattern 8.75 axles, not redrilled 1s.....ur small bolt backing plates r not the same..even if u drill the axles to big bolt...big bolt needs all big bolt components....kim.....
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Here's a picture of what I won-the axles don't look like they've been redrilled (just figured the bigger holes are for access to axle retainer bolts), but I'll check them when they come in of course-thanks for the reply. I planned to keep it all SBP for the time being, anyway. Swapping over to BBP will require a little more money to do than I want right now. I already have a pretty good amount earmarked on one of Mark's Dashes, and that'll require a couple hundred more on gauges and such to get installed. I also have to get my exhaust worked this week, then drop in my "new" engine, then front glass....
 

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I am doing this swap currently. I spoke with doctor diff the other day and he verified that yes, you can use the backing plates from the 7.25 if they are big bolt pattern axles. I have a '75 Scamp with discs up front and drums in the rear (5x4.5 bp). I will be using the whole brake setup from my 7.25 onto my new 8.75 housing.
 
Outer edge of 1 stud over to the center of the opposing stud.. Although I could be wrong... nothing unusual about that.
 
roadrunnerh said:
I am doing this swap currently. I spoke with doctor diff the other day and he verified that yes, you can use the backing plates from the 7.25 if they are big bolt pattern axles. I have a '75 Scamp with discs up front and drums in the rear (5x4.5 bp). I will be using the whole brake setup from my 7.25 onto my new 8.75 housing.
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Ok, so from what I gather, I cannot use the backing plates from my SBP 7.25" on the 8.75" assembly, right? If that's the case, are the brake parts compatible, or do I need to source new hardware, cylinders, shoes, and drums? What about the driveshaft-is it the same length? Is there anything else you can think of I'd need apart from a center section? My brother said he'd give me his old pig for the cost of shipping-741 with highway gears. I know it's not the big pinion 742 or 489, but it'll only be behind a stock 5.2 Magnum converted to 4bbl with a dual plane intake and 904.
 
My brother had a question about the Green bearings vs. stock Timkens, and I'm completely ignorant to them aside from knowing the Greens are sealed, and Timkens require periodic grease servicing [or so I'm told]. His concern was, since most aftermarket axle packages are sold with Green bearings, is there any likelihood of these axles departing under heavy cornering or high speed lane changes? It probably sounds like a bit of a dumb question, but I have no idea what to tell him.
 
Yes, the 7 1/4 SBP backing plates won't work on an SBP 8 3/4. The BBP 7 1/4 ones will fit if you have the BBP axles for the 8 3/4. The SBP 7 1/4 brakes will swap over to the SBP 8 3/4 if you get the correct backing plates. Driveshafts have to be shortened going from a 7 1/4 to 8 3/4. You'll need the correct, bigger U-bolts and shock plates too.
 
It has been said that the early Green bearings can't withstand heavy side loads and can fail. I have had the Greens in a 3800 lbs B body for 6 years with over 30,000 miles on them with no problem street driving.I know that they have improved/upgraded the Green since they first came out. I'm sure others will chime in with more info.
 
txstang84 said:
Ok, so from what I gather, I cannot use the backing plates from my SBP 7.25" on the 8.75" assembly, right? If that's the case, are the brake parts compatible, or do I need to source new hardware, cylinders, shoes, and drums? What about the driveshaft-is it the same length? Is there anything else you can think of I'd need apart from a center section? My brother said he'd give me his old pig for the cost of shipping-741 with highway gears. I know it's not the big pinion 742 or 489, but it'll only be behind a stock 5.2 Magnum converted to 4bbl with a dual plane intake and 904.
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The "green" bearings are a ball bearing whereas the stock bearings are a tapered roller bearing. The tapered bearing is a heavier duty design and will take more of a load before it fails. That being said, both bearings are held onto the axle shaft in the same manner.....pressed on with a pressed on lock ring. It's unlikely an axle shaft would escape with either setup but if it did, it would be because the axle slipped out of it's lock ring and bearing (both just as unlikely) or from catastophic bearing failure. In the latter case, it would be more llkely the green bearing would allow an axle to escape than a tapered roller bearing but I've never seen either one allow that to happen. You'll usually hear an axle bearing going bad long before it gets to the point of total failure.
 
cudamark said:
Yes, the 7 1/4 SBP backing plates won't work on an SBP 8 3/4. The BBP 7 1/4 ones will fit if you have the BBP axles for the 8 3/4. The SBP 7 1/4 brakes will swap over to the SBP 8 3/4 if you get the correct backing plates. Driveshafts have to be shortened going from a 7 1/4 to 8 3/4. You'll need the correct, bigger U-bolts and shock plates too.
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Got it...I think :scratch: Thanks! The axle is pictured above, and it shows backing plates-I would hope they are the correct ones for the axle. The seller said it was from a '68 Cuda; if those are the wrong ones and I have to go digging, will any SBP 8.75" backing plate be correct, or is there a different vehicle app that would work?

Luckily, we have a driveshaft shop in town-hopefully they can do what I need and supply the u bolts, joints, and yoke as needed. Shock plates I'll probably have to scour some classifieds.

Again-many thanks!
 
The backing plates are matched to a brake size and axle housing....not bolt pattern. The only ones that will fit have to come off an 8 3/4 rear with 10 X 1 3/4" brakes. That limits the donors to: A-bodies, and B-bodies with 6 cyl. engines and 10 X 1 3/4 brakes. Not a big pool to choose from. Your best bet is to find someone who has converted to the big bolt pattern and has the old backing plates still kicking around the shop.
 
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