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Projects Differential Identity

Discussion in 'Traditional Hot Rods' started by Powered by Ford, Oct 18, 2021.

  1. Powered by Ford
    Joined: Oct 18, 2021
    Posts: 31

    Powered by Ford

    Coupe Diff 8 bolt.jpg Coupe diff 2.jpg Hello, I'm looking for help in identifing the era of this earlier ford differential, in hopes to find out the gear ratio without opening it up (although I will have to replace the seal since it leaks a bit). Its currently in my 31 ford coupe. Im in the planning stage for this winters project. I'm rebuilding the 302 (79 block, 300hp+-) and looking to replace the auto 3 speed trany with an AOD. I would like to varify the third member so I know how to budget this project and have all three pieces working together correctly. Hoping to keep this rear end Coupe diff 2.jpg Coupe Diff 8 bolt.jpg intact, however if I need to replace the ring and pinion for the best set up, are they even available for this rear end? Thanks in advance guys.
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    FEL-PRO RDS55323 Info
    Cover Gasket; Rear; Ford; w/ 8 Bolt Holes; w/ 6.75 in Ring Gear
    fits 1979 Fairmont with 302 (and others)
    rear.jpg

    my guess is that the ratio is probably right for the transmission you already have, and you'll want different gears if you add overdrive (something around 3.5 ration instead of the high 2s you probably have now)

    and you'll want to put in a heavier rear end, just because you're one of those hot rodder guys.
     
    Hnstray likes this.
  3. Powered by Ford
    Joined: Oct 18, 2021
    Posts: 31

    Powered by Ford

    Thanks squirrel, I was able to find a little more info now knowing it's a 6.75. Still struggling to find other gear options available. I agree with using something in the 3.5 ratio. Hoping to keep the rear end. My rodding days are long passed. Reliability is what I'm after now. This is a 30 year old restoration project that I worked on with my dad back then. He recently passed and now I'm hoping to do it justice. We actually dug this car out of my grandfather's barn in the late 80's (see avatar). Wanting to make it safe and reliable again so the wife will want to ride along.
     
  4. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    If you want reliability, just reseal the transmission that's in it now, and keep the existing rearend. It's worked for 40+ years, it'll probably keep going a while longer.

    keep in mind that the 302 is a short stroke engine, and doesn't mind spinning 3000 rpm.
     
    1934coupe and Tman like this.

  5. Mart
    Joined: Mar 3, 2001
    Posts: 4,893

    Mart
    Member

    Ok, PBF, listen carefully.
    Jack up ONE back wheel. Mark the tire and a line on the ground next to it.
    Mark the driveshaft in a way that will allow you to count the turns.
    While observing the driveshaft, count the turns it makes while you turn the raised wheel exactly TWICE.
    The turns of the driveshaft equals the axle ratio.
    Mart.
    I show the principle in a video:
     
  6. Powered by Ford
    Joined: Oct 18, 2021
    Posts: 31

    Powered by Ford

    Liking the idea of keeping the transmission as well. It slips a little between second and third. Knowing it needs some love and after reading other discussions on the aod, it certainly got me thinking about replacing it. Still torn on that decision.

    Love the logic on the gear ratio and video, makes perfect sense. Sometimes we tend to overthink the obvious. Todays technologies sometimes cripples our ability to see things for what they really are. I probably would have thought of it 30 years ago when still working on cars from the earlier era where you could open the hood and still see the ground. That's why they are so gratifying to work on. Back to the basics. Thanks for the advice guys!
     
  7. Atwater Mike
    Joined: May 31, 2002
    Posts: 11,625

    Atwater Mike
    Member

    Sometime in the mid '50s, my '36 Ford 3 window had 3.54 rear, coupled to 25 tooth Zephyrs. Real tall takeoff, but at 35 MPH 2nd was pitched 'real close'...(ratio) High gear was very 'tall'...
    Gears can certainly handicap a young fellow! :eek:
     
    Powered by Ford likes this.
  8. I can't tell from the camera angle, but is there something bolted to the outside of the cover on the rear end? If so, that could lead to uneven clamping of the gasket, and a leak.
    If the car performed well with the current setup, I'd say just refresh and reseal the trans and differential, and drive it. If you are planning on driving it hard, rear end swap may be in the cards to handle more abuse. An AOD will help with hiway speeds and gas mileage, but: how much 75-80 mph driving will you be doing vs. how much gas can you buy for the $$$ it will cost to make the changes? How many miles a year will you drive it? I've thought about going to a GM version of overdrive automatic and different ratio in my rig, but even if I gain 5 mpg, it would take me quite a while to amortize it out.
    JMHO, YMMV
     
  9. 28phonebooth......good eyes mate.............that looks like the Panhard bar bracket which goes forward and has the Panhard bar located above the pumpkin, very short bar as well.........andyd
     
  10. Powered by Ford
    Joined: Oct 18, 2021
    Posts: 31

    Powered by Ford

    Yes, nice observation 28phonebooth. Andy dodge has it correct. It's a bracket for the pan hard bar.

    I can see that if the bolts holding the bracket are torqued differently it would cause a leak (squashing the gasket at those points). Or movement from the bar and braket bracket itself. I will certainly inspect the cover plate for flatness when replacing the gasket. Where would be a better place to attach the bracket? Maybe I should look for a stand off bolt that has threads beyond the normal head, so I can torque the cover bolt properly. Then attach the bracket with additional nuts. I suppose the movement could cause this again? Typing while I'm thinking here.

    Thanks for your additional thoughts 28phonebooth. I am leaning towards refreshing the trany more and more. I know it will all go back together without modifications that way. It's not the mpg that had me thinking and. I think is was more about the noise of the motor. Someday I'm going to drive it out of the city. Lol

    Regarding the rear end, thanks to the helpful tips earlier in this thread. I was able to check the ratio. 2 revolutions on the tire produced a little over 3 revolutions on the driveshaft (to about 1 o'clock) 25 (quater turn) divided by 3 is close to 8. So I feel it's a 3.08. Which I will probably leave well enough alone. Especially just refreshing the trany.

    I must say, I couldn't wait to test the gear ratio today after learning how to do it.
     
  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    I'd want to think that the same donor car gave up engine, trans and rear end and most likely the driveshaft.
    Engine noise probably will be more due to choice of mufflers than anything.
     
  12. Powered by Ford
    Joined: Oct 18, 2021
    Posts: 31

    Powered by Ford

    I actually got the 302 from a friend for my dad at the time. I don't remember where my dad got the the others. I was building a 460 Bronco at the time. Wished I was a little more hands on the model A back then.
     
  13. Ericnova72
    Joined: May 1, 2007
    Posts: 595

    Ericnova72
    Member
    from Michigan

    Rebuild the panhard bracket with longer ears, so it can go to 4 of the cover bolts. Then put the cover on with no gasket, just an RTV such as The Right Stuff so that there is no fiber gasket to squish and leak.

    Ideally?? The panhard bracket should be removed, and a new bracket welded to the tube a foot or more towards the driver side wheel, and make the panhard bar longer.
     
    Doublepumper and squirrel like this.

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