7 1/4 to 8 1/4 rear end

Transmissions and Rear Ends

  1. rcmaniac791

    rcmaniac791 Well-Known Member

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    I think that one of the last things I want to do to the fifth is replace that tiny 7 1/4 rear end with an 8 1/4 unit.

    **NOTE** Not doing this right now, or for several years to be honest, just looking to get some info and ideas

    I was looking on the allpar website to see what vehicles used the 8 1/4, and saw that 02-06 Jeep Liberties used it. I thought, "man, I used to have one of those!" So I know it pretty well. Those Jeeps are more and more common on craigslist or junkyards, so it seems like a good/cheap route. I also saw that the V6 manual trans comes with 3.55 gears, while the auto has 3.73 gears. The one I used to own was a V6 auto, and it was pretty peppy. Now I know why.

    Anyway, I know that the Jeeps have coil springs, as well as disk brakes in the rear. It would be cool for the 5th to have disk brakes on all 4 corners, so I suppose I would need a new master cylinder for that to work, right? Also, I'd have to address the springs and shocks mounts. If any of y'all have done this, what have you done?

    Like I said, not doing this for awhile. I like the current MPG's I'm getting. But eventually...
     
  2. BudW

    BudW Moderator Staff Member

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    You have a few options.

    Now is the time to gather parts – but I would wait until the existing 7¼” blows up, before changing it.

    Any 8¼” from any ’76-89 FMJ body will fit almost directly (driveshaft is slightly shorter).

    Any other Chrysler Corp 8¼” will fit - after modifications are made (differential perch and width are different so differentials may need to be cut down (or widened) to fit as well as perches cut off and re-welded onto tubes).

    The Jeep 8¼” with rear disk brakes is attractive – but is not a simple drop in (like another FMJ differential will).
    Our cars don’t need disk brakes in rear – but if I was going to modify a differential to fit, then why not go ahead with disk brakes as well.


    Now with that said, other things to consider is tossing the ISO clamps. One will need older shock plates or go with aftermarket ones (Firm Feel is one vender, there may be others).

    Another consideration might be to go with a bigger differential, like the 8¾. 9¼ or 9¾ (Dana).

    You can get a brand new 9¾” made to your exact specifications – which is nice (but it takes a person with more $, than I have).

    A ’66-70 B-body (’66-67 is preferred) 8¾” almost fits. Again the driveshaft will be shorter. The perch or shock plate will need slight modification because one differential is 0.44” wider than other one is, so both sides need to be offset 0.22” via grinding holes, or other means.

    Somethings to think about.

    Note: the most cost effective would be to find a FMJ 8¼” and its driveshaft, or cut your driveshaft down 1.6”.
     
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  3. rcmaniac791

    rcmaniac791 Well-Known Member

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    8 3/4 would be cool, but they are getting sort of hard to find, and the ones out there are expensive. If I'm going to spend a lot of money on a rear end, might as well go nuts and get the 9 3/4 like you said. I mean, the Dana 60 is so common that i could always get parts and support. I always like new parts, and by the time I do this, money won't be an issue.

    Do you know any good places that have these things? or can build one?

    Also, I'm by no means trying to be practical with it. the 5th won't be a daily any more when I swap the rear.
     
  4. BudW

    BudW Moderator Staff Member

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    There are at least two company’s making Dana 60’s new to your specifications: Strange and Moser. There may be others as well.

    The factory specifications are:
    Backing plate to backing plate 54.34”
    Drum to drum 58.50”
    Center of perch to center of perch (which is on bottom of differential) 44.46”.

    If you look at either website, you will notice you have more options to choose from, than you can count.
     
  5. rcmaniac791

    rcmaniac791 Well-Known Member

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    Cool. I'll check it out. Thanks for the help.
     
  6. Bruceynz

    Bruceynz Well-Known Member

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  7. Mr C

    Mr C Well-Known Member

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    If your 5th is stock, save your money and leave the 7.25 alone. It will last forever in stock form. Only consider this if you're upping the power.
     
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  8. BudW

    BudW Moderator Staff Member

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    I can not disagree with this comment!

    Having parts ready for a changeover is also nice - if you got room - for even the 7-1/4" ones are getting a bit more difificult to find.
     
  9. rcmaniac791

    rcmaniac791 Well-Known Member

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    i guess the big question here is if a different gear ratio is offered for the 7 1/4. That's the only real reason why I am interested in doing any of this work in the first place.
     
  10. BudW

    BudW Moderator Staff Member

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    What year is your current car - and I can tell you,
     
  11. rcmaniac791

    rcmaniac791 Well-Known Member

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    its a 1984 fifth avenue
     
  12. rcmaniac791

    rcmaniac791 Well-Known Member

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    I think I may have struck gold:

    guy on my local craigslist is selling an 8 3/4 out of a truck apparently with 3.23 gears for $275!! its in good shape too. Brakes, everything is there. He says it's 56 1/2 inches from backing plate to backing plate. Would that fit the 5th or would I have to modify it?
     
  13. brotherGood

    brotherGood Well-Known Member

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    You'll have to modify a truck axle..but if that's all you've gotta do..you're ahead of where some are. There's a debate as to either going that route, or the b body route, Jeep, Ford 8.8..etc. that's what hot rodding is about though IMO..take what comes and make it work. If you've got someone who can narrow the tubes and axle shafts..you're sitting pretty good
     
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  14. BudW

    BudW Moderator Staff Member

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    The (stock) FMJ differentials are:
    58.500" drum to drum (where wheel meets the drum)
    54.340” flange to flange (backing plate to backing plate)
    44.460” center of perch to center of perch
    The perches are located on bottom side of tube.
    Trucks generally have perches on top side of tubes.

    That differential is 2-3/16” wider (1-3/32” per side) – so depending on wheels, might not stick out too much. Perches will need work, though.

    ’69 C-bodies and all E-body’s (neither came with an 8¼”) are 56.437” flange to flange & 46” perch center.

    I’d like to know which pickup that came out of?

    BudW
     
    Last edited: Oct 3, 2016
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  15. 80mirada

    80mirada Well-Known Member

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    8 3/4" were made until at least the end of 74 model year. Half ton trucks and vans used them during that time, not sure when they ceased being used or what options they were included with.
     
  16. BudW

    BudW Moderator Staff Member

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    My bad. I just now saw on post a few lines up does mention 8¾” (was thinking 8¼”).

    ’65-71 Dodge D100 8¾” specs are (not A100 or B100):
    (Rear) 63.940” Drum to drum
    58.312” Flange to flange
    Perch to perch center (not yet known)

    ’72-74 Dodge D100 8¾” specs (again, not A100 or B100)
    (Rear) 64.380” Drum to drum
    59.437” Flange to flange
    43.500” Perch center to perch center

    If the differential rcmaniac791 is from something else – I would like to get the specs from it, for my records (please).

    BudW

    Edit: the perches are on top, on both examples given above.

    4*4 pickups from '65-74 - the front differential is also 8¾”, but ring and pinion were made to turn the opposite direction.
    There is not much a front 4*4 differential that will fit - other than the spider gears and assorted fasteners.
     
    Last edited: Oct 3, 2016
  17. rcmaniac791

    rcmaniac791 Well-Known Member

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    Here's the Ad. for the 8 3/4 out of the truck. Claims it's a '70: Mopar 8 3/4 rear complete

    Doing some more snooping, I found another one in a bit worse shape, but it's out of a B-body. The seller is not sure if it's either a '64 or '65. He only wants $100 for that. Mopar 8 3/4 B-Body Rear end


    Also wanted to add that I may end up not going with the truck rear-end, seems like more modification than I want to get into.
     
  18. 89.Fifth

    89.Fifth Well-Known Member

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    Truck one you're gonna have to replace the axles, drums, and who knows what else. Trucks were 4.75 bolt pattern so your existing wheels won't fit.
     
  19. brotherGood

    brotherGood Well-Known Member

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    Depends on the year from what I understand. I think they were the same until 85 or so..then the truck got a different bolt pattern.

    I'm almost sure that's what I was told..because I wanted to put my AHB wheels on my 89, and I'm almost certain that's what was said to me.
     
  20. 80mirada

    80mirada Well-Known Member

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    Half tons are 5x4.5" until 85 or 86 I think, and them go to 5x5". 4.75" is GM