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Identifying Differential
#31
Yes, there are some concerns about it. There's a reason that Ford used the 31 spline 9 inch differentials behind high performance engines, and the 28 spline 9 inch behind 351Cs.

It will partly depend on the rear end ratio and the tires you will be using. Large diameter and width tires hook up better, more traction means more load on the differential and the rest of the drive train. Another factor will be the shape and diameter of the axle shafts, some of the 28 spline axles do not maintain the same diameter for the full length, but narrow down between the splines and the hub. A low gear ratio (numerically high) differential has smaller pinion gears which also means weaker.

If I were running that engine I would definitely run a 31 spline 9 inch, either stock or aftermarket like Currie or Moser. And, if I were planning on frequent trips to the drag strip step up to 35 spline axles.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#32
(01-21-2020, 09:55 AM)Don C Wrote: Yes, there are some concerns about it. There's a reason that Ford used the 31 spline 9 inch differentials behind high performance engines, and the 28 spline 9 inch behind 351Cs.

Hey Don , How ya doin'?

Here is my version of Darth Vader's 2.75 Nut-sack  (another "2" in the VIN)

[Image: under.jpg]

N
ow, it is a 9" ...(my wife doesnt think I have 9 inches, but WTF! looks like 9 to me  Tongue )

 ...My bag of bolts, as you know, is an April 1971 Dearborn "02" body, "M" code 351C std 4V with FMX trans, and a grotesque guy behind the wheel 

[Image: eng1.jpg]

How would a numpty like me know if it is 31 spline or 28 ?


plz be gentle  whistling

[Image: siggy.jpg]

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#33
If stock it was a 28 spline. Which is fine for what you are running.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason


[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
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#34
The best way to be 100% sure is to remove an axle and count the splines.

However, you can be 99% sure by looking at the end of the axle. In this picture the axle on the left, with the Double "D" shaped slot in the center of the hub, is a 28 spline axle, and the one on the right with the 3 holes in a row is a 31 spline. Now, the reason for the 1% doubt, 8" axles all have 28 splines and most have the 3 holes in a row. An 8" axle of the same length will fit in a 9" housing.
[Image: 28vs31spline.jpg]



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#35
IF you decide to go look for another 9" rear axle, here are the dimensions you would be looking for taken from mine. Sorry I don't have a fancy drawing program to make it look all neat and tidy, but you get the idea. Between the inside of the brake backplates measured 53 1/4" and the between hole centers on the mount plates was 43" (approx, just a quick measurement) I believe there were several axles with these dimensions used on other Ford vehicles, possibly Bronco, F150's etc. I'm sure Don C or Don from OMS can confirm, but there should be options out there, not just Mustangs. I'm going to suggest you order, or buy from a vendor like NPD, Kevin Marti's book, The Mustang and Cougar Tagbook (1965-1973). In it you will find just about every tag listed for our cars. There are way too many axle tags for me to list here. Someone may already have a complied list of axles and cross references they can post for you.
Geoff.

[Image: IMG-1043-2-LI.jpg]

upload image

Geoff.
 I learn something new every day!
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#36
(01-21-2020, 11:22 AM)Don C Wrote: The best way to be 100% sure is to remove an axle and count the splines.

However, you can be 99% sure by looking at the end of the axle. In this picture the axle on the left, with the Double "D" shaped slot in the center of the hub, is a 28 spline axle, and the one on the right with the 3 holes in a row is a 31 spline. Now, the reason for the 1% doubt, 8" axles all have 28 splines and most have the 3 holes in a row. An 8" axle of the same length will fit in a 9" housing.
[Image: 28vs31spline.jpg]

This is probably a dumb question, but if I get a new 9" differential, and with that new axles, I'm guessing that I will also have to get new wheel hubs (or the new axle will have it attached) and I will need new wheels, tires (tyres for you wierdies out there), and drum brakes (unless I upgrade to all disc) for this new configuration?
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#37
The hub is part of the axle. You get the bolt pattern you need for your wheels, which for stock wheels is 5x4.5" (5 lugs on a 4.5" [114.3mm] diameter}. You'll likely need to get new backing plates for drum brakes (unless your new axle also has small bearings), good time to upgrade to discs. You'll also need to get the correct width, which is 61.25" from outside of hub to outside of hub.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#38
(01-21-2020, 01:57 PM)Don C Wrote: The hub is part of the axle. You get the bolt pattern you need for your wheels, which for stock wheels is 5x4.5" (5 lugs on a 4.5" [114.3mm] diameter}. You'll likely need to get new backing plates for drum brakes (unless your new axle also has small bearings), good time to upgrade to discs. You'll also need to get the correct width, which is 61.25" from outside of hub to outside of hub.
Whats the potential damage if I don't swap to a stronger differential and all the extras that requires right away? If I'm not too hard on it, should it hold up, or am I sitting on a ticking time bomb?
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#39
(01-23-2020, 07:31 AM)DarthRevan39 Wrote:
(01-21-2020, 01:57 PM)Don C Wrote: The hub is part of the axle. You get the bolt pattern you need for your wheels, which for stock wheels is 5x4.5" (5 lugs on a 4.5" [114.3mm] diameter}. You'll likely need to get new backing plates for drum brakes (unless your new axle also has small bearings), good time to upgrade to discs. You'll also need to get the correct width, which is 61.25" from outside of hub to outside of hub.
Whats the potential damage if I don't swap to a stronger differential and all the extras that requires right away? If I'm not too hard on it, should it hold up, or am I sitting on a ticking time bomb?

I beat the living be-jeezus out of an all original 3.00 8" behind a stout 4 speed 351C for a couple years. Nothing ever broke, because traction was always an issue. If you put sticky tires on it, you're going to eventually break it. 

IMO, drive it and enjoy it, just make a plan for a 31 spline 9" upgrade for the future.


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