FMJ Differentials

Transmissions and Rear Ends

  1. BudW

    BudW Moderator Staff Member

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    All (’76-89) FMJ-body differentials are interchangeable – with a couple of minor differences.

    The ’76-81 FMJ 7¼” came with 2.5” (63.5 mm) tube diameters.
    The 7¼” differential tube diameter bumped up to 3.0” for ’82-89 M and J bodies. The ’82-89 7¼” and all FMJ 8¼” use the same shock plates (3.0” (76.2 mm) axle tube). The 76-81 7¼” differentials use a different shock plates (both parts/both sides - see red circle, below) because the axle tube diameter is smaller.
    If you are replacing a 7¼” with an 8¼” into a ’76-81 FMJ, you will also need all of the shock plate parts as well.

    Unrelated, but it would also be a great time to just upgrade the ISO-Clamp system to the older style shock plate to begin with.
    Iso-Clamp.JPG
    Red circle is ISO-clamp (has 2 large black rubber pads) and Blue circle is the older style shock plate (no rubber pads).

    A quick way to tell if you have an ’82-89 7¼” differential is the 3.0” (76.2 mm) tubes neck down to 2.5” (63.5 mm) just before entering the center section and looks something like this:
    7.25 Diff Identification.png


    The ’76 to mid-year (early) ’80 FMJs have 58.5” (1,486 mm) track.
    Chrysler considered track to be “mid-tire to mid-tire”. I believe it might be the same as WMS (Wheel Mounting Surface) to WMS – but not measured it to verify.
    The late ’80 to ’89 have a 59.5” (1,511 mm) track (each side is 1/2” (12.7 mm) wider).

    Both track sizes will interchange – but if you are using aftermarket wheels, you might want to check on clearance between tire-and-leaf spring as well as tire-to-fenderwell before calling the repair “done”.

    To recap, the five (5) different differentials that came in (all) FMJ’s are:
    - ’76 to mid-year ’80 7¼”, 2.5” (63.5 mm) tube, 58.5” (1,486 mm) track, has a 9-bolt cover.
    - Mid-year ’80 to 81 7¼”, 2.5” (63.5 mm) tube, 59.5” (1,511 mm) track., has a 9-bolt cover.
    - ’82-89 7¼”, 3.0” (76.2 mm) tube, 59.5” (1,511 mm) track, has a 10-bolt sorta diamond shaped cover.
    - ’76 to mid-year '80 8¼”, 3.0” (76.2 mm) tube, 58.5” (1,486 mm) track, has a 10-bolt oval cover.
    - Mid-year ’80 to 81 8¼”, 3.0” (76.2 mm) tube, 59.5” (1,511 mm) track., has a 10-bolt oval over.

    The propeller shaft length is approximately 1.6” (40.6 mm) longer for the 7¼” differential OR 1.6” (40.6 mm) shorter for the 8¼” differential – depending on which way you look at it.

    An example of a 8¼” (taken by a fellow forum member)
    8.25 Differential.jpg

    8¼” cover gasket
    63-12 8.25 RDS55047.jpg

    7¼” Cover Gasket
    '76-81
    76-81 7.25 RDS12775.jpg

    '82-89
    82-96 7.25 RDS55392.jpg
    BudW

    Note: the cover gasket pictures were "borrowed" from the internet.
     
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  2. BudW

    BudW Moderator Staff Member

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    Installing a Non-FMJ Chrysler differential into an FMJ body.

    FMJ’s all have the spring perches at 44.46” (center to center). The vehicle closest to that measurement is the ’65-70 B-body, which has spring perch centers at 44.0” (0.23” difference per side – which is enough to force the spring into compliance without any issues).

    Chrysler publishes the rear wheel track (center of tread to center of tread) – but doesn’t specify the WMS (Wheel Mounting Surface) distance or flange to flange measurements.

    Official Tread distance:
    '65-67 B-body is 58.5” (1,486 mm)
    '68-69 B-body is 59.2” (1,504 mm)
    1970 B-body is 58.7” (1,491 mm)
    The ’76 to mid-year ’80 FMJ is 58.5” (1,486 mm)
    Mid-year ’80-89 FMJ is 59.5” (1,511 mm)

    All other Chrysler differentials will either need to be cut to size and will need perches welded into correct locations.

    All 8¾” differentials came with a 3” (76.2 mm) tube diameter.

    The propeller shaft length will be approximately 0.66” (16.8 mm) shorter between 8¾” from an 8¼” differential OR 2.26” (57.4 mm) shorter between a between 8¾” from a 7¼” differential.

    If considering installing an ’65-70 B-body 8¾” differential, then I would also highly recommend using the shock plates from the same vehicle.
    shock plate.jpg

    There are other options as well, like this ISO delete kit sold by Firm Feel:
    FFI ISO delete kit.jpg

    The ISO-Clamps used on FMJ’s do not readily fit onto the B-body differential without a lot of work. The FMJ leaf spring centering bolt “head/nut” diameter is larger than what was on the B-body leaf springs. I recommend getting a pair of C-claims (to hold leaf spring together), remove your existing centering bolt and replace it with a new pair of B-body center bolts OR take a drill and drill out the perch holes and shock plate holes to fit.
    The rear shocks attach to the shock plate a bit differently – but just get some ’66-70 B-body shocks and you are in business (which should be easier to find than FMJ rear shocks are).

    Not a good picture, but the leaf spring centering bolts are on the bottom row center of the (just above) picture.
    BudW
     
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  3. volare 77

    volare 77 Well-Known Member

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    I have a 8 1/4 that I assume is out of the 80-89 group as it is a inch wider then the 77 F body 8 1/4. One thing I noticed was the spring perches were about a 1/2 inch off from the 77 volare rear perches.
     
  4. BudW

    BudW Moderator Staff Member

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    Chrysler did use 7¼”, 8¼”, 8¾” and 9¾” (Dana 60) differentials in the ’66-70 B-bodies, in the various widths mentioned above.
    The 7¼ were only used for /6 only (’68-70) and 8¼” was used for 318’s only (’69-70) – and any of those will (also) fit into an FMJ body without issues (other than the tube diameter on 7¼” and propeller shaft length).

    I’ve installed a few B-body 8¼” and 8¾” into F-bodies. It is not worth the effort with the B-body 7¼”, for it is just as flimsy as the FMJ versions are. Even back in the late '70's and 80's, it was hard to find a "good" used FMJ 8¼” differential and "an already trashed out" 7¼” were easy to find, even back then - so a lot of people used a B-body differential to keep driving a disabled car.

    I would bet the differential you had was from an '69-70 B-body replacement (for a 7¼”). I’m not aware of any date codes on outside of differentials. A person would have to pull the inspection cover and look at dates stamped on the ring gear to know for sure.

    The only thing good about the ‘60’s to early ‘70’s 7¼” (A and B-bodies) are they do have decent ring and pinion gear sets in them (like 3.23’s, for example). The 60’s to early ‘70’s 7¼” (A & B-bodies) only came in /6 cars. That changed with FMJ-bodies when they put the 7¼” behind the 318 and 360’s (and what were they thinking?).

    The 8¾” differential is popular and is still supported by aftermarket – so gear sets (etc.) are still available and with a lot wider range of gear sets (and availability) than the 7¼” & 8¼” parts are, no matter what year it was made in.
    Unless you have a strong engine – it is difficult to damage an 8¾”. If you do have a car with a healthy engine (Like Duke5a, for example) then the ’66-70 B-body Dana 60 will fit right in - but I consider that overkill for the other 99% of us.
    The Dana 60 is about as strong as it gets.

    I have no experience with other brand differentials to say anything good (or bad) about them or what it might take to install an off-brand differential into an FMJ.
    BudW
     
  5. 80mirada

    80mirada Well-Known Member

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    8.25 was used behind 383-2v in 1970, possibly in 69 as well
     
  6. slant6billy

    slant6billy Well-Known Member

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    My 77 has a one legger 7 1/4. My plan is to put a sway bar and spring from a AHB dip. the 7 1/4 is going by by. 8 1/4 with a jeep 3.55 or 3.73 sure grip. I figure the super 6 with the OD trans will have a nice track attack
     
  7. BudW

    BudW Moderator Staff Member

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    I forgot to mention. The ’62-24 B-body differential has close to the same measurements to the ’65-70 B-body differentials – but the axles and wheel bearings are completely different.
    iuHVTAHX60.jpg
    Instead of a slide off brake drum (like what FMJ’s use), they use a press-on brake drum with a tapered axle shaft.
    64 rear drum removal.jpg
    63 Tapered Axles.jpg
    A design I hope I NEVER work on again. I believe the housing ends are different from the ’65 and newer ends, as well.


    The ’71-81 B and R-body differentials have a 47.3 inch (1,201 mm) center-to center perch distance. Also, the differential is just too wide for FMJ (about 5 inches (127 mm) wider).
    ’71-74 8¾” differentials go for big bucks and that crowd goes crazy when they find one.

    ’63-76 A-body differential perches are 43.0 inch (1,092 mm) wide and overall about 3-4 inch (76-102 mm) to narrow for us.
    Also, the '63-72 A-body uses 5x4.0" pattern wheels.

    ’65-74 C and E-body perches are 46 inches (1,168 mm) apart and width varies between 60.7 to 62.4 inch (1,542 to 1,585 mm) wide, depending on the year.
    Who would have thought that C and E-bodies share the same differential measurements?

    The ’75-78 C-body differentials share the same measurements as ’71-79 B-body, except for station wagons which is a couple inches wider.

    The ’65-73 Y-body (Imperial) share the same perch measurement as ’65-74 C/E-body, at 46 inches (1,168 mm) apart, but width is wider. Width varies between 61.1 to 63.4 inch (1,552 to 1,610 mm). Y-body also uses a 5x5.5” bolt pattern wheels and not the 5x4.5” that FMJ’s use.

    There is a chance that A and B-Vans as well as pickups might fit – I just don’t have measurements for them.
    B-vans and pickups have the perches located on top of differential, where the A-vans and cars have perches located on bottom if differential.
    BudW
     
  8. Opticon77

    Opticon77 Well-Known Member

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    Last edited: May 13, 2019
  9. BudW

    BudW Moderator Staff Member

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    To clarify, Chrysler 7¼” 8¼” and 9¼” differentials uses a straight wheel bearing.
    8.25 9.25 Axle Bgn.png
    This the wheel/axle bearing for an 8¼” or a 9¼” (the same bearing fits both differentials). The inner bearing race contacts the axle shaft itself. Sometimes the bearing surface eats away at the axle shaft itself (at location marked by white arrow).
    8.25 Axle.jpg
    If it will catch a fingernail, then the bearing is shot and needs to be replaced. The axle can be reused by using an offset bearing like this one:
    8.25 9.25 Axle Repair Bgn.png
    These offset bearings have only been out for about 10-years now (or so) and not something that was available when I worked at a dealership.


    On the 8¾” and 9¾” (Dana 60) the axles use tapered bearing.
    iuR6YPOR1Y.jpg
    Recently, many drag racers are using “Green” aka straight wheel/axle bearings, for the frictional loses are less for straight line racing.
    For street use or for circle racing, many people believe they are not the best because the “Green” bearings can’t carry a side load like the tapered bearings can. I tend to agree with the later “though” process - but am not going to change someone’s mind if they are going to consider using ”green” bearings.

    Now with that said, the post # 7 (third picture down) shows a tapered axle shaft (which also has tapered bearings pressed onto it).
    The older stile tapered axles will not accept “Green” stile bearings, from what I’ve heard.
    All I know is if you have a car with tapered axles, after working with them some time, you will be looking at replacing them - for they are a huge pain in the ___!
    BudW
     
  10. Oldiron440

    Oldiron440 Well-Known Member

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    I have installed more aftermarket axles than I could ever remember and they have all had the sealed green type bearings. As for as not holding up as well as the tapered roller bearings I believe that to be true because most everything now has the sealed bearings and they seam to start failing at 100K including the ones on the rear of my Expedition.
     
  11. ZieglerSpeed

    ZieglerSpeed Well-Known Member

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    I need a little help from my friends. The trouble I'm having is removing the differential cross pin bolt on a 8 1/4 rear. Is it locktited? Will it work to heat the head of the bolt (5/16 wrench) with a propane torch? Must I use acetylene torch? -Thank you, -Tim Z
     
  12. ZieglerSpeed

    ZieglerSpeed Well-Known Member

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    Still hoping somebody will give me their 2 cents worth on weather heat will help on my differential cross shaft bolt
     
  13. XfbodyX

    XfbodyX Well-Known Member

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    What do you have to heat it, a normal ox/ac torch with a rosebud tip? If so and id heat just below the bolt head on the cast, thats where the threads are.

    You might even try a propane torch.

    I just read you only have a propane torch, id try it on the cast area of the carrier vs the bolt head.

    Id almost try to shock it loose with a box end wrench and a tiny mallet. Some can be a pain but they always come out.

    This is a ebay pic, but you can see where your threads will be.

    bolt.jpg
     
    Last edited: Jun 21, 2019
  14. BudW

    BudW Moderator Staff Member

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    Shock (using a 2-pound sledge or compatible) and using a high quality closed-end end wrench are the two keys to loosening that pin bolt, for Loctite red is what is holding that pin in place (and not bolt torque, per se).
    BudW
     
  15. ZieglerSpeed

    ZieglerSpeed Well-Known Member

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    If I get in trouble...I see Yukon Gear has a broken bolt extractor kit that has good reviews. No reason to think I'll break the bolt, just adding information