View Full Version : Rear-End Rebuild: What Is the Best Way to Go

12-06-2005, 08:21 PM
The good news is that I found out that my '96 Cobra has 4.10:1 differential gearing; the bad news is that the rear-end is in for a good rebuilding. :mad:

In your view, what are the worst- and best-case scenarios for repairing a Cobra rear-end that occasionally sounds like an old school bus whining down a long hill? :)

Suppose that I had to replace both rear axles and the differential, would that be a $2K-plus adventure with new differential gears and two new axles? And, are there kits or refurbished rear axles that might be a smart way to go if one or both of the axles is/are horked?

I realize that it is hard to really tell without taking the the differential apart and looking inside, but should I listen for any particular noises in relation to a bad rear axle versus bad differential gearing, etc.?

And, what is the best differential kit available for the '96 4.6 Modular Cobra?

(...I guess that covers it.) :D

Thanks for your help.
--Professor Chaos

12-06-2005, 10:20 PM
Here's one option, I guess, but I prefer 4.10:1 gearing and hope to find it in an all-in-wonder setup like this:

Mustangs Unlimited 8.8-inch Axle Assembly (http://www.mustangsunlimited.com/itemdy00.asp?T1=M4006B373+01)

(Note that the above item is an "assembly" (whatever that means) and that it "is fully assembled with all applicable brackets, gear set and 28-tooth Trac-Lok" but apparently still "requires 28-tooth axle shafts", which are another $256.95):

Ford Racing Hi-Po Axle Set (http://www.mustangsunlimited.com/itemdy00.asp?T1=M4235D+01)

Total $1076.90 less labor, for 3.73:1 assembly plus axle set

12-06-2005, 11:47 PM
You can purchase a new differential and axles for less.
I would take it to a shop and have them look at it, or look at it yourself before puchasing anything, but if anything is messed up. You should be able to just replace or rebuild the differentail any gears, which would be less than $1000 for the parts.

If you want to replace it all, just go with Moser 31 spline axles, 31 spline locker, and some 4:10 FMS gears. Have it professionally installed and be done with it :D

12-07-2005, 07:49 AM
If it is just noise you hear, and unless you really want to change the ratio, than have a look at the gear lash in the pinion ring gear. Thats most likely your problem and can save you a whole lot of money to fix just this problem. At the same time change your axle seals just for kicks. But if you find badly worn gears it could be one of many problems, the biggest being the rear end housing could be bent only slightly (hard to see by eye) and setting a new set of gears into a bent housing means you will be doing the change again soon after. Just giving you my opionion from experience. Been racing for 30 years now.:thumbsup:

12-07-2005, 08:54 PM
Thanks for the suggestions and for sharing from your extensive racing experience. It is just noise at this point: I haven't noticed anything unusual in the way that the differential engages with the drive shaft, just that the rear-end sounds like a bearing is about to spin-out or like an axle is bent and thus rubbing slightly against the rear-end housing. Also, I notice that the noise seems to be more pronounced toward the outer portion of the driver's-side axle, but the noise might just be emanating from the point to which the axle housing mounts at the chassis.

I'll look into diagnostics and at that time ask my mechanic (an SVT-certified tech) what might be the best course of action, both in the short- and the long-term. That will hopefully enable a better-educated approach, one that avoids throwing good money at bad hardware. It might be better--if the differential and axles will hold-out until March--to wait untl then (when I will have cash flow) and then "shotgun" (completely rebuild) the rear-end; it might be best to go for the best thing that I can build (4.10:1 and all), knowing that Lorena the Cobra is a high-mileage unit (with 113K) and thus deserves diligence during rebuiling. (The suspension is finished; the drive train is next.) :D

Thanks again for your help.
--Professor Chaos