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Technical Differential question

Discussion in 'Traditional Hot Rods' started by blazedogs, Oct 20, 2014.

  1. blazedogs
    Joined: Sep 22, 2014
    Posts: 488


    Was thinking about going with a 8" or 9" Ford rear end. The car is a 1929 Ford Model A Coupe.Will be fenderless. Have been told there are some issues to consider ,certain years are getting hard to find parts for ,The overall length has to fit the car and look right ,will have accomidate the right brakes etc, Would like your thoughts ?

    Also ,what ratio would you consider ? What about ( 3.0 ) Is a very light car...Will ( not) be running large ,wide tires in the rear.

    Gene Farmington, Mn
  2. blazedogs
    Joined: Sep 22, 2014
    Posts: 488


  3. blazedogs
    Joined: Sep 22, 2014
    Posts: 488


    Forgot to mention, Motor is a Small block Chev Basically stock 4 barrel Carb ,Mild cam,headers,no high compression heads.
  4. I ran a 3.00 gear in an 8 inch under a 2800 LB ford with a mild smallblock and a 350 turbo. Could not have been a better setup for street/hiway combination driving. Can't help you with parts availability problems. I thought everything was available for 8 and 9 inch ford rears.

  5. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 9,098


    Ford explorer 8.8". If you shorten one axle tube (driver's side) and use another short axle, it'll be about right width. My 97 Explorer is 3.73:1, 31 splines with traction lock and disc brakes. Swap the OEM tin cover and upgrade to an alloy rear cover. The 3.73 is good with an OD transmission.

    OEM track, wheel mounting to wheel mounting surface width: 59.5"
    Drivers side axle shaft length: 30.688"
    Passenger side axle shaft length: 27.813"
    Axle shaft length difference: 2.875"

    This means cutting 2.875" out of the drivers/long side axle tube, resulting in an overall axle width of about 56.5". Conventional pinion flanges and other ratios are readily available. Plenty of information out there,
  6. Texas57
    Joined: Oct 21, 2012
    Posts: 3,450

    1. 1952-59 Ford Social Group

    It looks like you deleted your querry on the gear ratio, but for that, the guys in the know will probably want to know what tranny you're running and what your planned usage of the car is.
    As far as parts availability, in view of the fact that the Ford 9" has got to be the most popular differential ever, and there are lots of manufacturers/suppliers for just about anything you would want for them, I would think parts availability would be the last thing you'd need to worry about.
    There is lots of info on the net about them, such as which ones you should look for if it's to be modified (shortened, etc), which cars came with them and what the length is from those particular vehicles. I believe the '57 Ford passenger cars have about the shortest oem width.
  7. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,925


    The only parts that are getting hard to find are early brake stuff. It's all out there, just not stacked up like cord wood, like the old days.
  8. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,856


    The 9" has much more stuff available in the aftermarket at better prices than the 8".
  9. pitman
    Joined: May 14, 2006
    Posts: 5,151


    Running top gear, 3.08 ratio with one-to-one.
    (Actually most efficient since it's a straight pass through)
    If over driven as alternate choice, then between 3.43 and 3.73 .
    Last edited: Oct 21, 2014
  10. If bolt pattern is a concern, the Ford truck 9-inch have 5 x 5.5 pattern, same as early Ford. Either 8 or 9-inch Ford are strong enough for the mild SBC you plan to run, with 9-inch being stronger. An 8.8 is also a choice, most are 5 x 4.5 BP; and strength is between 8 and 9-inch. Most 8-inch and most passenger car 9-inch are 5 x 4.5 BP. Width for the rear is what will look best for your car, it may likely require narrowing the donor rear you find.

    I agree for a nice driver, a 3.0 gear with a non-OD trans and decent tall rear tires is a good combo. Your mild engine will pull the gear fine.
  11. Fenders
    Joined: Sep 8, 2007
    Posts: 3,921


    2 wheel drive S-10 rear axle is the perfect width, only issue would be the Chev bolt pattern.

    Attached Files:

  12. You can order up a 9" rear any way could imagine and have it all 100% brand new including the housing.

    I think the 8.8 is a great rear end. Its a heavy bastard though. Shortening the exploder axle is relatively easy. Another nice thing is that you can mix and match all sorts of parts out of them.

    The only trouble is the axle tubes are 3.25 OD and that means most of the ready to go brackets and stuff won't fit. Not a problem if you can make simple brackets
  13. The early Broncos are a good choice for a Model A.

    58 inches...1966-1977. HRP
  14. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,800

    Atwater Mike

    About the "two short axle" formula: Be advised that axles (even 'fat ones') take a set;
    Simply replacing a long right hand axle shaft with a short lefty will tend to 'unwind' the molecules as there is heavy force applied.
    My younger brother removed the axles from his '29 Rdstr/P.U. ('54 Olds rear) and mused the way the splines were 'twisted'. He told a story that they were 'machined that way'...I pointed out that the 90 weight gear oil leakage at both backing plates were from the axles twisting and moving outward...both backing plates were bent at their mounting circles.
    The splines were twisted ONE FULL TOOTH WIDTH.
    'Machined that way'. My brother was machined that way.
  15. Plus you get to keep the big bolt pattern. HRP

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