Putting 11" drums from 8 3/4" onto 8 1/4"

Transmissions and Rear Ends

  1. MoparKidD-4

    MoparKidD-4 Well-Known Member

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    I have an 8 3/4" housing with 11" brakes in great shape just sitting in my garage. I'm getting an 8 1/4" ready to swap into my '88 5th Avenue and I thought it had 11" brakes but it doesn't (those big finned drums fooled me, shoulda measured or at least looked at the rim of the drum lol). Can I just swap over the entire brake assemblies from that 8 3/4" on? Seems like a no-brainer except for that fifth backing plate stud which the smaller rear ends don't have (they only have 4 studs holding the backing plates on but they have the same spacing as the 8 3/4").
     
  2. F body Deconstructor Jim

    F body Deconstructor Jim Well-Known Member

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    You sure about the 4 stud deal?
    I just pulled a 7.25 from a 79 F body and it had 5.
    I dont recall taking apart anything with less than five studs on any F or M body.
    The earlier A body 7.25's only had 4.

    Those finned drums are the heavy duty 10" set up. Wagons came with them.

    Virtually any post 1965 5 on 4.5" rear drum brake assemblies will fit any post 1965 mopar rear housing with the exception of 72 and older A body rear ends. They use a different backing plate offset.
    5 on 4.5" bolt pattern 7.25, 8.25, 8.75, Dana all interchange.
    Be sure to swap the whole assemblies.
     
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  3. MoparKidD-4

    MoparKidD-4 Well-Known Member

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    Yup I'm sure, the 7.25 under my '88 5th Ave has 4 studs, so does this 8.25 which is out of a '90s Jeep Cherokee... might be a late thing?
     
  4. F body Deconstructor Jim

    F body Deconstructor Jim Well-Known Member

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    the 88 dippy 7.25 I parted had 5.
    No clue on anything past 89.
    I know there were a few changes but I have enough trouble remembering the stuff I do.:confused::D
     
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  5. BudW

    BudW Moderator Staff Member

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    I have seen a couple of 4 stud differentials – but most (well, all but a couple) were 5. All of the backing plates I have seen for these cars, are 5-hole plates.

    It seems that someone was either lazy about the 5th stud, or lack of sufficient parts on assembly line warranted them to use less studs, maybe.
    I don’t know. I guess one had to be there, to know for sure.

    I don’t think I would worry about the lack of the 5th stud.
    BudW
     
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  6. AJ/FormS

    AJ/FormS Well-Known Member

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    So are you still putting 11s onto the 88 5thAve? Cuz, I think you will have trouble proportioning those with the OEM valve.
     
  7. MoparKidD-4

    MoparKidD-4 Well-Known Member

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    Yeah I'm planning on it... I was thinking about proportioning but this is a pretty heavy car with pretty even weight distribution I'd imagine. If it's really bad I'll add an adjustable valve but I also plan to upgrade the front brakes to the big B/R-body 12" rotors and performance pads at some point.

    It's kind of scary thinking this car weighs about 600 lbs. more than my '70 Duster but has essentially the exact same brakes (Duster has '73-up BBP brakes), save for power assist on the 5th Ave. The Duster can stop on a dime but I've run out of brakes exiting the freeway loaded down with 3 adults in my 5th Ave... not fun lol.
     
  8. BudW

    BudW Moderator Staff Member

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    AJ/FormS has a point.
    You do not want the rear brakes working harder than front brakes, or you will have an issue of car swapping ends (or directions) during heavy braking!
    11” drum brakes with 11” front brake rotors will call an imbalance in front/rear braking – without use of the proper brake proportioning valve (or add-on adjustable valve).

    If you upgraded front brakes to the B/R 11¾” front brake rotors, then I think you would be fine – but I would upgrade front brakes, first.

    I do have plans on going 11¾” front rotors and 11” rear drums on both cars, and have 95% of brake parts ready to install. My plan is do both (front and rear) at same time – but waiting to finish rebuilding my 8¼” differential (new bearings and seals only), first (do it all at once).
    BudW
     
  9. AJ/FormS

    AJ/FormS Well-Known Member

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    Keep in mind that the rears were designed to do about 15% of the braking duties, on a car with 4 same size tires on it.
    If the rears lock up in a turn, you will be spinning.
    If the rears lock up first, you are milliseconds from being in a turn, and then spinning.

    My car has the 11" KH-4 piston jobs up front with 235/60-14s.
    On the rear are 10x2s with 295/50-15s and zero proportioning.
    There is no tendency to lock the rears, yet it is the best stopping car I have ever owned. The rear is doing a lot of work. I am at 3650 me in it. I can't get anybody to ride along with me anymore, they all complain about whiplash, the sissys :)
    If you put on 11s with the same size tires as are on the front, I hope your Life Insurance Policy is up to date. Those 11s will want big tires like at the races.

    If you want more rear brakes with the current 10s there are better options;
    1) adjusting the damn things to come on properly. Many times the rears are doing nothing at all cuz when you step on it, the front discs work right away so you stop pushing. Meanwhile, in the back, the shoes are not even touching the drums.
    1B) Sometimes the front sliders seize up, and then you may be braking with just 1 pad on the front. Or say 1.5 pads, or they will be wearing crooked, or the guy before you wore the steel backer right off and a part of the piston too, and now the darn piston is cocking in the bore on every application.Or they could be el-cheapo pads. Or the discs could be heavily grooved. Or the hoses are acting like little expansion chambers, or ..... well you get the picture. The front brakes are your lifeline, you need to make sure they are in perfect operating condition; starting with new hoses and fresh fluid.
    2) more aggresive rear brake linings
    3) Fresh turned not-bell-mouthed drums, with custom ground matching radius linings.
    4) larger wheel cylinders. I think there are 3 sizes available.
    5) an adjustable proportioning valve, to send more line pressure to the back. The factory proportions the rear operating pressure to about 75% of the front discs.


    Most of the above may require larger rear tires to NOT lock up prematurely. More height works in a straight line. More width works better in corners. I have the 295s for traction in corners. 275s were enough for straightline work.
     
    Last edited: May 17, 2017
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  10. BudW

    BudW Moderator Staff Member

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    One does not want rear brakes to work too hard, for if they do lock up, the rear of car will be swung around – if not into an uncontrollable spin – before you know what happened.
    IT IS A BIG DEAL!

    Logic (what my head thinks) is if front brakes lockup, and rear does nothing then that would cause rear end to come around – but in actuality, that does not happen. It goes against what my head is saying logic.
    BudW
     
  11. Oldiron440

    Oldiron440 Well-Known Member

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    I put 11" drums on my 76 Volare with no other changes and never had an issue with break performance. I've had a wide array of wheel a tire combinations on the car in colluding 30×10.5 drag slicks and BFG TAs. The swap was done in the late 80s and are still on the car. I'm one of those guys that sandbag when they bracet race so I use the brakes hard at times @ 125 + mph..
     
  12. AJ/FormS

    AJ/FormS Well-Known Member

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    Like I said; 11s need big tires, like at the races. And if you have not defeated the P-valve then you may not have accomplished as much as you think.

    But I agree, at the big end you sometimes need a lot of brakes
     
  13. Oldiron440

    Oldiron440 Well-Known Member

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    All four BFGs were all the same size, balance was not affected..
     
  14. MoparKidD-4

    MoparKidD-4 Well-Known Member

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    I have 275/60-15s on the back of this car, I'd kind of rather have 255s but anyway... I am aware of the dangers of rear-wheel lockup, try driving an old RWD Mopar in 8 inches of snow a few times and you'll learn the braking dynamics of your car REAL quick lol. I know upgrading rears before fronts may upset the balance but if that's the case I will just do the mods to correct it sooner than later.

    TBH the size of the rear drums doesn't matter a whole lot with the annoying self-energizing design that is guaranteed to lock up if you hit the pedal too fast. Even the small drums will lock with an inexperienced driver in a panic stop. You have to adjust them to that fine line between premature lockup and not working at all lol.

    Oh and I also want the 10" brakes off my 5th Ave to swap onto my '93 Jeep Cherokee that has teeny-weenie 9" drums.
     
  15. F body Deconstructor Jim

    F body Deconstructor Jim Well-Known Member

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    OK...here's one...
    just finished off a 78 four door, slant six 904.
    It had an 8.25...with 4 bolts on each backing plate to axle flange.
    The plate had five holes. The flange was only partial.
    I've never seen one like it. OK...one that I can remember:D
    Also its a C clip axle. I hate those but maybe its why there are only 4 bolts. 100_0796.JPG
     
  16. AJ/FormS

    AJ/FormS Well-Known Member

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    Lock up 9" rears?
    I've never seen that happen, even with 13inch tires back there,lol!
     
  17. Kernel Sanders

    Kernel Sanders Well-Known Member

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    IIRC all 8.25's and 9.25's are C-Clip DJ