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Discussion Starter · #1 ·
So here I am again. Building my Lincoln Mark 3. Got my engine finally ready. It is a 521 and will probably be in the 600-650 hp range with a lot of low end torque. Transmission will either be a upgraded C6 or 6R80 with adapterplate and standalone, not sure yet.

Now, my car has the stock 9 3/8 rear end. I've been told it is a pretty beefy rear end, but I figured it probably won't do the job (for long) with this amount of power. I only found some rebuild kits for this rear end. I read the 9" is much easier to upgrade.

What would be the best thing to do? Stay with the stock 3/8 as is? Do a overhaul on it and hope for the best? Or get me a 9" performance part and some custom axles with it?
 

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Discussion Starter · #3 ·
9 3/8" is weak. Go with the 9" and new axles.
Alright, was afraid I had to do this. Did some reading on the internet, if I'm not mistaken, then a 9" center section would fit in the 9 3/8 housing, as the 9 3/8 is just a notched 9. Can anyone confirm if this is right? Then I could keep my stock axles right? What would be a good gear ratio? Was thinking of around 3-3.25
 

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Alright, was afraid I had to do this. Did some reading on the internet, if I'm not mistaken, then a 9" center section would fit in the 9 3/8 housing, as the 9 3/8 is just a notched 9. Can anyone confirm if this is right? Then I could keep my stock axles right? What would be a good gear ratio? Was thinking of around 3-3.25
It is just a notched housing. The axles will work, but they're old and not the best for high HP/TQ heavy vehicle setup. You'll break them sooner or later. If you go to a 9" which will fit, get a 31 spline one along with new 31 spline axles. Piece of mind.
 

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My 2¢ worth:
The 9-3/8" third member is found in early Lincoln's and other large ford vehicles it is more heavy duty than the standard 9”.

The 9-3/8 Positives : A more robust casting but is not nodular, A larger differential carrier that is likely to be limited slip, 4 Pinion gears, 31 Spline spider gear and axles, Larger bearings throughout, My research says the 9-3/8” is a strong 3rd member stronger than a 9”.

The Bad: 9-3/8 are much heavier than the 9", Parts are hard to find altho the limited slip clutches are the same as the 9”, Gearing options are limited around 3.25 ?? to 2.80 ?? or so.

Common 9” 3rd members will interchange directly with the 9-3/8 3rd member. You must have 31 spline spider gears. The yoke can be switched from one to the other.

The 9-3/8 3rd member will fit common 9” banjos provided they have a cutout same as most gaskets for the larger ring gear. Its interesting that most gaskets still today have clearance for the 9-3/8 ring gear. Few have any idea why it is there.


I think the reason Ford discontinued the 9-3/8 is they were overkill stronger than necessary. Combined with the reduced horsepower and gas crisis of the early 1970’s it did not make sense to continue with them.

I run two different 3rd members in my 557 cid 72 Mustang. For street use I run the 9-3/8 Limited slip with 2.8 gears. For other stuff i run a 9” with Detroit locker and 3.5 gears.

 
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