7.25 to 8 3/4 conversion

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Andrew vanman87

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On my 71 duster I currently have a 7 and 1/4 rear I purchased an a body 8 and 3/4 with backing plates for and 8 3/4... ( With new shock plates)It's currently a slant six car with a 904... One day I would like to put a 340 in it but that's not for another year or two out and wanted to do a few upgrades along the way... I was trying to figure out what I would all need to do the upgrade I currently have competition leaf springs already in the car..

Probably by new u-bolts

I'll need new brake lines

My primary question is the drive shaft that's currently in the car able to work or hook right up to the 8 and 3/4 ?
Will it needs shortened?

Does the 7.25 rear ( drive shaft )have
Have the 7260 u-joint that would hook up to my 8 and 3/4 yolk?
Will the brake t work on my 8 and 3/4?

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Yes to new u-bolts. The 8.75 will have 10 x 1.75 drums which may be bigger than the slant’s front drums ( probably 9 in) — from experience I can tell you this does not work well. Yes to a new driveshaft, but you could have yours shortened.
 
See somebody already did it a disc brake conversion to the front of the car it's large bolt pattern and for the rear in order to get large bolt pattern rims to fit on.. they put these spacers on them to accommodate it... Which I'd like to get rid of... These are the brake backing plates that I have for the 8 and 3/4.... But I wasn't sure about the drive shaft in my mind I would think it would have to be shortened... But if I'm going to need a new drive shaft anyway or shorten the one I have I'm leaning towards just making a large u-joint drive shaft for my car for when it's ready for the 340

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Oh, and I don’t recall having to do anything to the e-brake cable.
 
The driveshaft definitely has to be shorter. The snout of the 8.75 is longer.

The 10 x 1.75 rear brakes are a good match for the discs up front — that’s not a problem.

A-body 8.75 rears are all small bolt pattern, meaning you will need new axles to match your big pattern in front (BTW, this is why I chose Kelsey-Hays discs when I did a conversion — to retain small pattern all around so I wouldn’t have to also buy new wheels and tires). But the other argument is that going big bolt pattern will give you a better choice of wheels in the future.
 
Yea I was going to buy the big bolt pattern axles from doctor diff... and I would have never put the spacer it's how I bought the car looking get rid of them as soon as possible ... think I am just going to get a new driveshaft made... ( Large u joints )

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I had the change them right a way... they are the mancini competition springs... the reason for the urgency on changing those first was because the original slant 6 springs where so weak the tires rubbed if I had more than 1 person in the car ...

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Do you know if the brake backing plates that you have for the 8 3/4 came from a car with big bolt pattern axles? There is an offset difference between the small bolt pattern 8 3/4 backing plates and the big bolt pattern ones. Dr. Diff can tell you how to tell the difference. Your brake T should bolt back onto the 8 3/4, and the vented bolt that holds it should be the same threads as the new rear. If you get lucky, it will be in the same location as the 7 1/4 and the hard brake lines will work back. If not, just make new hard lines. Try to find a set of U bolts with the flats on the bottom side of the curve at the top. Completely round bolts will crush the axle tubes. Dr. Diff may be able to fix you up with those too. Another thing to consider, is if you are going to upgrade to a 727 when you install a 340, you would need to have your driveshaft cut yet again. The 7 1/4 will most likely have the 7260 U joint. You can measure both rear end yokes to see. If the 8 3/4 yoke is a 7290, you can buy a u joint that is 7290 on one side and 7260 on the other to adapt to your existing driveshaft after it is shortened.
 
well i was told they came off a challenger ( but i was not sure why i made the post ) my goal is to run large bolt pattern axles as far is the transmission goes i run 904s in all my cars so that wont be a issue ...I am sold on just having a new one made ( drive shaft) ( with the large u joints ) 7290 my 8 3/4 center section has a large yoke already . i am currently building a 340 stroker (416) but wanted to drive the car till I am ready to drop it in. i did order the u bolts from doc diff ( but i will check if the are flat at the tops of them )
 
If the backing plates came off of a Challenger, they will work fine with Dr. Diff's BBP axles for an A body rear end. 10-4 on the driveshaft. I wasn't sure if a 727 may be a future possibility is why I threw that part in. A good 904 works better in a hot rod anyway! I've used the flip flop u joint before with no problems or vibration if you just want to shorten the existing driveshaft for now to not have to pop for a whole new one right away. Hope it turns out really good for you!

:thumbsup: :thumbsup:
 
Just out of curiosity, why would axles that simply have a different lug pattern use anything different than stock A-body brakes???
 
The wheel mounting hub sticks out farther on the BBP axles than a factory 8 3/4 A body axle. You have to swap brakes when you add BBP axles to an original A body 8 3/4. If you re-drill original A body 8 3/4 axles, and also re-drill the brake drums to match, then the original brakes will work.
 
JDMopar said:
If the backing plates came off of a Challenger, they will work fine with Dr. Diff's BBP axles for an A body rear end. 10-4 on the driveshaft. I wasn't sure if a 727 may be a future possibility is why I threw that part in. A good 904 works better in a hot rod anyway! I've used the flip flop u joint before with no problems or vibration if you just want to shorten the existing driveshaft for now to not have to pop for a whole new one right away. Hope it turns out really good for you!

:thumbsup: :thumbsup:
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well that what the old guy i bout them off told me but ill take some pic send them to doc diff and see what he says about them .. if they really did come from a challenger they should work
 
str12-340 said:
Just out of curiosity, why would axles that simply have a different lug pattern use anything different than stock A-body brakes???
Click to expand...
oh they made small bolt pattern and large... ( many A bodies are small bolt pattern ) my duster has been upgraded to large bolt pattern... well sort of lol when bought the car the guy put large bolt pattern front disk on the car and in order to fit large bolt rims on my car he put a wheel spacer on the the back of the 7.25 so he could put the large bolt rims one the car .... i am looking to get rid of that rear and put large bolt axles on the car ( trying to do it the right way) why mopar did the back and forth between large and small patterns ... i am not sure on that one ..
 
Just going to repeat this: A-body 8.75 rears were ALL small bolt pattern. When A-body cars went to large bolt pattern, they stopped using 8.75s and instead used 8.25s on V8 cars. So it is not going to be an A-body 8.75 AND have LBP axles UNLESS it had been converted by a previous owner. B-body and E-body 8.75s ALWAYS had large bolt pattern. But the axle lengths are different from A-body.
 
Yeah I never had the original axles to this a body 8 and 3/4... But I imagine they were small bolt pattern... Which makes sense because every a body 8 3/4 I've seen has been small bolt pattern... so I definitely believe ya I'm ordering a set of axles from Dr diff green bearing large bolt pattern to run the large bolt pattern rims and get rid of those weird wheel spacers
 
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