question about the 7.25

Transmissions and Rear Ends

  1. picklesgarage

    picklesgarage Well-Known Member

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    yes I get it they are junk. everything I can find about them says the weak spots are the carrier bearings and spider gears in non posi's. is it worth the time/money/effort to bother upgrading one to be a posi and changing the gear set? seems like there are carriers and gears galore on ebay for them. has anyone bothered to do such? Im not planning going crazy with my 86 5th ave, only engine mod im planning is a 4 barrel swap. plan is in the spring for this to become my daily but its a snoozer with the stock gears. im not finding any 8.25s in the north east, or at least any yards the will respond back to my inquiries. the 8.75 seems like over kill for a stock 318 let alone my budget. thoughts? idea? input? thanks
     
  2. Duke5A

    Duke5A Well-Known Member

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    These cars are simply too heavy for the 7.25". See what Uncle Tony is doing with a /6 drag car and a 7.25". He is banking on it surviving because the car only weighs 2700 lbs. Your Fifth Avenue weighs ~4200 lbs with fluids and a driver. It's a junk axle.

    Look at the Ford 8.8 swap already done here. Shorten one side and get a combo u-joint. Done. Should be very budget friendly.
     
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  3. picklesgarage

    picklesgarage Well-Known Member

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    Duke5A. that was something i hadnt considered about these rears, the car just being too heavy. most of the discussion i could find about the 7.25 was on A-body forums and some people chiming in about E-bodies. there was a write up about using a liberty 8.25 but i dont remember there being any mention of the width or clearance issues
     
  4. Aspen500

    Aspen500 Well-Known Member

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    My Aspen originally had a 7.25" attached to an E24 (90hp) S6 and 4 speed O/D, with 72,000 miles and,,,,,,it broke the spider gears one day taking off from a signal light. BANG, coast around corner onto a side street, all done, ride home with tow truck driver. No, it wasn't operator induced by abuse either! :eek: Can't imagine one of those behind a 318.
     
  5. BudW

    BudW Moderator Staff Member

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    The 7¼” first came out in ’60 (or ’61) for a low-cost differential for the /6 A-body (only) – and was used until end of production in ‘76. Later (late '60's), it was used for 318 2-bbl A-bodies as well.
    The other cars Chrysler used the 7¼” was:
    - ’68-70 B-bodies /6 only (note: these will also fit FMJ’s with little effort).
    - '70 E-body /6 (very few of these got made).
    - ’76-89 FMJ-bodies (/6 or 318)
    - Early Dakota pickups with 2.5L 4-cyl or 3.9L V6, 2*4 or 4*4 (note: the front differential ring and pinion gear set are in reverse direction (cut wise) than what rear differentials use).
    Many of the pickups use 3.55 or 3.91 gear sets because of the taller tires – which does open up possibilities – but make sure you look at picture of the gear set before purchasing because the front (4*4) gear set will not work – because of the reverse cut of the gears.

    I looked this up a long time ago, but Chrysler offered limited slip’s in 7¼”s for only three years (going by memory) and even then, very few selected that option.
    A Limited slip was available aftermarket for a while (in the ‘80’s) – but I hadn’t seen one for sale in a long time.

    Gear sets I found on eBay are: 2.2, 2.4, 2.7, 2.9, 3.2, 3.5, 3.9 and 4.1 (I left off the last digit, for it sometimes varies).

    If you already have a 2.2 or a 2.4 – any of the other gear sets (except for 2.7) will wake the car up.
    2.9 would be the highest I would go. 3.2 is a great combo gearset (town and highway). 3.5 would be fun for a in-town car. 4.1 might be a bit low for most people. 2.2 and 2.4 gear sets are just TOO HIGH to get any power out of a car.
    When talking about differentials, a higher ratio means lower numerical number. A lower ratio is a higher numerical number.

    This is an (aftermarket) ring gear and pinion for a Chrysler 7¼”.
    C7.25-2.93.jpg

    This a ring gear/pinion set for a Chevy 7¼” front differential. Yes, I know comparing apples to oranges, but the reverse cut pattern is the same as the Dodge 7¼” front differential. The ring gear would bolt to carrier and pinion would fit into the housing – but it would be impossible to get the ring gear/carrier to fit into the housing with that pinion gear already in it.
    GM 7.25 IFS.jpg

    The 7¼” can handle a mildly warmed up 318 4-bbl – without too much problem, but with that said - I’ve had to work on tons of these differentials with /6’s in them, back in the day. The entire differential (not just one part) is just not that strong.

    My recommendation has been (still is) to go out and have fun but have (or be actively looking for) an 8¼” stuck under the garage work bench for it does blow. I had an 8¼” (sale pending) for this very reason, but just acquired a pair of 8¾” differentials to upgrade with (for my big blocks). I agree an 8¾” is a bit of overkill for most of us.
    An 8¼” is a stout differential and is still being used in light trucks and Jeeps today.
    BudW
     
  6. BudW

    BudW Moderator Staff Member

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    I have been gathering data (an on/off again affair) to see what current differentials will fit in our cars.

    FMJ cars have two different width differentials
    ’76 to mid-year ’80 are WMS (Wheel Mounting Surface) to WMS is 58.50”.
    The mid-year ’80 to ’89’s WMS to WMS are 59.50”.
    The spring perch center to center are 44.46”.
    The distance from center of axle to center of U-joint are: 10.09” for 7¼” and 11.69” for 8¼”

    The ’02-07 Jeep Liberty (KJ) rear differential WMS to WMS are 63.38” – which is 4” wider (2” per side). This differential doesn’t use leaf springs, so existing coil springs and brackets will need to be cut/ground off and spring perches purchased and welded on.

    ’93-98 Grand Cherokee (ZJ) rear WMS are 60.88” – which is roughly 1” wider. This also has coil springs on it. This uses either 8¼” or Dana 44 differentials. The D44 is 8½”. Some of the XJ D44’s use aluminum housings.

    ’84-01 Jeep Cherokees (XJ) rear differential WMS to WMS are 60.63” – which is also 1“ wider. The spring perches are located on top instead of bottom, so again, they will need to be cut/ground off and new perches purchased and welded on. These came in either Dana 35 (7½”) or 8¼” versions. The D35 is stronger than the Chrysler 7¼” and has a lot more parts availability for them.

    ’87-96 Dakota (rear) is WMS-WMS is 62.63” – but most of them have 6x4.5” wheels – which is not fun to have two different kinds of wheels on the car.

    I’m still looking into other options but sometimes my time is allocated elsewhere.
    BudW
     
  7. Aspen500

    Aspen500 Well-Known Member

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    I wonder if the Gen 1 Dakotas all have an 8 1/4" rear? Pretty sure '87-'91 are 5 lug 4.5" circle while Gen 2, ('92-'96) are the stupid 6 lug 4.25" circle. I say stupid because I own one and other than stock wheels are basically unobtainium. Actually the rear in my 4x2 '96 Dakota is called an 8 3/8" but sure looks the same as the old 8 1/4". Suppose one bonus to the Dakota rear is most have 3.55 or 3.73 gears already (mine is 3.55). Maybe not a bonus without an overdrive though.
     
  8. BudW

    BudW Moderator Staff Member

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    Most (or many?) of the Gen 1 Dakotas use 7¼” diffs. I don’t know when they changed from 5x4.5” to 6x4.5” bolt pattern wheels. It might be at start of Gen 2 (IDK).
    I really dislike the 6x4.5” wheels. I mean why go that way. I never saw any reason to do it.
    It's been a long time sense I've seen a 5x4.5” Dakota.

    I think that most (all?) of the 8-1/4” are actually 8-3/8” in diameter
    BudW
     
  9. AJ/FormS

    AJ/FormS Well-Known Member

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    I agree.
    Three things kill that diff;
    an overheated cross pin as in peg-legging ,
    and shock-loading as in drag-strip starts,
    and old,tired, contaminated, oil.
    The spider gears, cross-pin, and case, will not put up with that type of abuse for long.
    In your Fifth,with a stock TC, good luck with getting her to squawk a tire, much less peg-leg it. That leaves Neutral-Drops. Stay away from those, and freshen the oil, you could be fine for decades.
    On a personal note, I have had good success even with a 340,and a warmed 318,by observing those two rules. If by some miracle your Fifth does spin, install a SG, and carry on. When both tires spin together,in a straight line, the rear is actually not working that hard.
     
  10. Aspen500

    Aspen500 Well-Known Member

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    Maybe it's (8 1/4" and 8 3/8") similar to the Ford 3.8L which became a 3.9L when the Freestar replace Windstar in 2004. What is the difference? Absolutely nothing, exact same bore and stroke.
    Little Dakota has six, 1/2" wheel studs, big 4x4 full size Ram has five, 1/2" wheel studs. Makes no sense at all, he says going off OP topic (sorry). Yes, Gen 2 (starting in '92) is when the stupid 6 lug came about. Would love to meet the engineers and ask them,,,,,,,,,,,,WHY?:confused:
     
  11. AJ/FormS

    AJ/FormS Well-Known Member

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    me too!
     
  12. picklesgarage

    picklesgarage Well-Known Member

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    the 05 Dakotas to end of production in 12 went back to 5 lug and rear drum
     
  13. picklesgarage

    picklesgarage Well-Known Member

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    Seem like the hunt is still on. I will not have a back up car this summer where im starting the rebuild for my charger. im not overly gentle on most of my cars and the idea of babying just makes me want to get rid of it. im sure there is one out there somewhere for sale
     
  14. picklesgarage

    picklesgarage Well-Known Member

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    are the 7.25 considered the square cover and 8.25 round?
     
  15. BudW

    BudW Moderator Staff Member

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    The 8-1/4” (or 8-3/8” depending on who you talk too) has always had an oval cover (with 10-bolts).
    8.25 63-12 RDS55047.jpg

    The 7¼” was always called a 9-bolt cover – but Chrysler changed the cover to 10-bolts in ’82 – which tossed that statement out the window.
    7.25 60-81 RDS12775.jpg
    ‘61ish to 81 cover (9-bolt)
    7.25 82-96 RDS55392.jpg
    ’82-‘96ish cover (10-bolt)
    Just not sure what shape you will want to call the 7¼”s - but not oval.
    BudW
     
  16. picklesgarage

    picklesgarage Well-Known Member

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    I would agree and call the 8.25 oval. all the salvage yard list them as either square or round. not the moat helpful description when both are "round". would think the fact that the 7.25 axle housings taper going into the center would be better
     
  17. Aspen500

    Aspen500 Well-Known Member

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    Thee's always the odd balls. My dad's '74 Gold Duster (or what's left of it that is, major rot) is a 225, 3 speed floor shift manual, manual everything actually, but has an 8 1/4" rear. With the 3 speed and 2.41 gears, it's doesn't have ANY traction problems out of the hole, lol! You'd think a car like that would have been built with a 7 1/4" rear,

    There are plenty of FMJ 8 1/4" out there. The problem is, it's something that almost needs to be sourced locally (similar to an engine or trans) since shipping would be by truck and that means $$$$.
    Most salvage yards are on a network and can find parts and have them shipped to the yard so the cost isn't as high. Have you checked into that route?
     
  18. picklesgarage

    picklesgarage Well-Known Member

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    Aspen500 been trying yards, most don't seem to want to respond or are not open on weekends. im sure some of the lack of response is that I am emailing them since not really supposed to be on the phone during work, not supposed to be on here/ email either. bit easier to hide then off on a phone call. I remember yards used to always be open on the weekends and there were more of them.
     
  19. 80mirada

    80mirada Well-Known Member

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    the 8.25 became the 8 3/8 when they enlarged the ring gear and pinion gear head slightly.
     
  20. BudW

    BudW Moderator Staff Member

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    7.25 Diff Identification.png
    This picture only applies to 10-bolt cover 7¼” (with 3" tapered axle tubes - '82-89).
    BudW
     
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