8 3/4 swap questions...

Brakes for your Classic Mopar

  1. Sublime69

    Sublime69 Well-Known Member

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    Hey guys, this is a LOOOOOONG over due project but i currently have a 1974 7 1/4 (BBP) rear axle in my 69 Dart. I have an A body 8 3/4 (SBP)A body rear axle that i want to swap into it. My question is, do all i have to do it get big bolt pattern axles and drums? Also is the drum width thicker on the 8 3/4? My wheels have minimal clearance with the 7 1/4 so just seeing what im in for here. I have also read that the A body 7 1/4 and 8 3/4 are the same width from flange to flange. Is that correct?

    Thanks!
     
  2. dano

    dano Evil Handy Man

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    The 8-3/4 will get wider with the LBP axles and brakes I just can't recall how much. If the rims are close now then they will not fit. Off the top of my head an 8" wide rim in the rear will want at least 5.75" or more of backspace. You'll need to measure. takeoff 94-04 Mustang rims seem to work well here. What do you have up front as the 73-up make the front wider too.

    You will need to address the driveshaft.
     
  3. Sublime69

    Sublime69 Well-Known Member

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    Do you know if a disc brake conversion will keep the over all width the same? or at least closer to my 7 1/4 width?
     
  4. dano

    dano Evil Handy Man

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    It would actually get a little wider as rotor flanges are thicker then drums. Some had drilled the stock axle and drum to LBP. I wouldn't. Do you have custom wheels your trying to reuse?
     
  5. Sublime69

    Sublime69 Well-Known Member

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    Yes, i guess my only other option will be to have the housing narrowed.
     
  6. 4mulas

    4mulas Fixem'up

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    I’ve done this and know a bit about it. When putting a small bolt pattern A body 8 3/4” diff into a car or converting your existing small bolt pattern 8.75” diff to large bolt pattern you need to:

    1- Order and use new axles with LBP, doctor diff has them New with green bearings installed for $300. Make sure to measure your diff from flange to flange to ensure it is an A body axle. I think the proper measurement is 52 5/8” from flange to to flange if memory serves.

    2- next, you need rear brake backing plates for any mopar that uses 10” x 2.5” rear brakes. Junkyard ones work here from cars like: 73-76 dart and duster with 10 x2.5”, newer rear wheel drive dodge diplomat’s, rwd Chrysler New Yorkers/fifth ave’s, any mopar with 10x2.5” brakes. Grab the spreader bar and e-brake lever’s too. What ever you pull the backing plates from is what you’ll order your shoes and drums for. These parts are readily available, check rock auto.
    These plates compensate for the different offset others have mentioned

    3- in your case you may have to change your rear rubber flex line/brake hose if you find the 7.25” diff hose too short/long. See if you can order one that would have come with 340/383/440 darts as those cars would have come with an 8.75” diff.

    4- you probably need to shorten your drive shaft and perhaps have a different rear u joint set up put on the shaft dependant upon which yolk you have on your Center section of the 8.75” pumpkin.

    5- seeing as you’re going heavy duty now, you may want to look at your rear leaf springs to make sure they are up to the task too.

    6- your steel brake lines from the 7.25” may not work for you on the new diff, have a look at those too.

    7- hard to tell from your avatar but if you have positive offset wheels on your you could order longer studs on your axles from doctor diff and use a wheel spacer to accommodate the positive offset wheels. Then you don’t have to spring re-locate. I’d do up to an inch spacer Max, Not ideal, but doable. Maybe post a picture of your wheels.

    8- install all this and giver!
     
  7. 4mulas

    4mulas Fixem'up

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    U bolts. Probably need new u bolts too....
     
  8. Sublime69

    Sublime69 Well-Known Member

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    Thanks for the info! So I should be able to use the backing plates and brake set up from the current 7 1/4 rear axel thats in the car currently
    ?
     
  9. 4mulas

    4mulas Fixem'up

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    No, probably not as they are likely 9”. If they are 10 they’d only be 10 x 1.75” at best but 10” is unlikely on that diff.

    if you don’t want to chase junkyard parts, call dr. Diff as he can sell you everything you need brand new...
     
  10. Jimacuda

    Jimacuda Well-Known Member

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    He has a 74 big bolt 7.25. It should have 10x2-1/2" brakes.

    10x1.75 and smaller was small bolt only, if memory serves...
     
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    • dano

      dano Evil Handy Man

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      I think any BBP will work. I have 11x2.5" from a C-body or truck or something.

      If your looking to narrow it to fit the wheels then start with a different 8-3/4. I wouldn't cut up an original A-body one.

      What is your current wheel offset/backspacing?
       
    • 4mulas

      4mulas Fixem'up

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      Oops, sorry!, totally right. Misread that one. Yes he should have the right backing plates and hardware

      and yes you’re also right, 10 x1.75 was small bolt 8 3/4”. Small bolt 7 1/4” usually got 9”
       
    • 4mulas

      4mulas Fixem'up

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      yes, those 11” ones would fit too but may be to big for the a body dependant in what he’s doing for front brakes.

      YES- dont cut up an A body 8.75” diff!! I asked him to post a pic of the wheels he has earlier. I ha e a feeling he’ll be fine but we need to see them.
       
    • MOPAROFFICIAL

      MOPAROFFICIAL Well-Known Member

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      Some a bodies like '71 swinger , possible significance had Kelsey front disc, had "10 x 2 or drums on it.
       
    • Dartnut

      Dartnut Don't hate me because i'm beautiful

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      If it's a true BBP 7 1/4 rear end, it'll be out of a '73 and up car and was transplanted in at some point in the car's past.
      It will also have the 10''X 2 1/2'' brakes and backing plates on it already, and you can reuse them on your 8 3/4.
      It is also wider than a pre '73 SBP a-body 7 1/4 rear end, i'm not sure by how much, but it is wider.
      You will need shock plates and u-bolts for the 8 3/4 size which has 3'' axle tubes.
      You will probably need the brake hose and lines for an 8 3/4, plus your drive shaft will need to be shortened.
      I would suggest that you get ahold of Dr.Diff for the axles etc. that you need, he is great to work with and his prices and service are very good.
      This all being said, the a-body 8 3/4 is narrower than a BBP 7 1/4, but when you put BBP axles in the 8 3/4, it in effect widens it so your wheels will be close to where you are now as far as clearance.
      I hope this helps...........
       
    • Jimacuda

      Jimacuda Well-Known Member

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      Never seen 10x2 small bolt pattern rear brakes.

      A body BB cars got 1.75" rear brakes
       
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      • Sublime69

        Sublime69 Well-Known Member

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        Here is the only decent pic of my rear wheels.

        A679A03C-82A3-4ABF-B9C8-7460746CE1FB.jpeg
         
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        • 4mulas

          4mulas Fixem'up

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          True, but they also usually Had a 8 3/4 diff too. Like a scamp with a trailer towing package. Small bolt 2” does exist but is pretty rare. Funny how the 340 cars often got 1.75” but a 318 with tow pkg got 2”
           
        • 4mulas

          4mulas Fixem'up

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          nice car. If you could measure the backspacing then would be ideal. At a first glance I’d like say you should be okay with the conversion.
           
        • DentalDart

          DentalDart Well-Known Member

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          Good luck! I had roughly the same clearance on both wheels/tires and wheel wells when the 7 1/4 was installed. Just dropped the rear end to the ground over the weekend with new tires (same size as previous) and the 8 3/4 installed. My drivers side tire has between 3/4-1in clearance and my passenger rear tire has 3/16-1/4in clearance to the wheel well. I kept drum brakes in the rear just went from 10in rear drums to 11 rear drums.

          20200112_144451.jpg

          20200112_144555.jpg
           
        • 72bluNblu

          72bluNblu FABO Gold Member FABO Gold Member

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          One really important thing to clean up- a BBP 7.25 axle is NARROWER than an A-body 8 3/4 with BBP axles. Flange to flange a BBP 7.25 is only 51.5”. Mounting surface to mounting surface is 56.75”, the BBP A-body 8 3/4 will be 57 13/16”. If you’re talking about adding disks you add another 5/16” per side.

          Which means wherever your wheels are now, they’re gonna move out over 3/4” per side.
           
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          • 66fs

            66fs Well-Known Member

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            Get a custom narrowed 8 3/4 from Moser. They know the right length to cut a housing and the axles to match it.
             
          • 72bluNblu

            72bluNblu FABO Gold Member FABO Gold Member

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            Or just get rims with the right backspace for a helluva lot less.
             
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            • 66fs

              66fs Well-Known Member

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              To match the 5 x 4.50 bolt circle on the front discs, I was shortening a housing anyway, so it was a no brainer to do it right and be done. They know exactly what lengths to make the housing and the axles. What is the cost for a trouble free rear over 30 years?
               
            • MOPAROFFICIAL

              MOPAROFFICIAL Well-Known Member

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              It was 7 1/4 though , sorry Iol I meant to state that...n that was the point in the 1st place.
               
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