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Discussion Starter · #1 ·
Hi everyone,

yesterday my 70 Mustang's rear end started to make an unnerving grinding noise (the one you don't want to hear coming from any machinery) while driving slowly. I lifted the car and saw that the driver's side rear wheel was covered in oily, stinking substance. Since the brake cylinder is fine and the oil smells like rear end (you know what I mean), my assumption is that some seal or bearing resigned.

The problem is, that I don't know which kind of rear diff I have.

Here is what (I think) I already measured and know: I got 28 Splines, Small Ford Axle Housing Flange with 2 1/2" Offset, Total Width 59", Housing Width 54", Wheel Bolt Pattern 5 x 4 1/2". What I don't know is, if this is an 8" or 9" Ford Diff or something completely different. Based on the pics below, if any of you has some insight into which kind of diff I have, please let me know.

Thank you all!
 

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Looks like a 9", probably the original axle for the car since the it appears to be a 428" car. Seeing how wet the axle are, I was looking for the vent hose on the pics but can't see it?
 

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It’s a 9 inch. Looks like the pinion seal has been leaking for a while. You might want to service that if you pull the center section from the housing. How is the oil level? You may have run it low on oil causing a wheel bearing to overheat. You have to pull the axles to check the bearings anyhow so I would pull the center while I was at it and attend to the pinion. Strange that a 428 car would only have 28 spline axles. By the way, 8 inch brakes, backing plates and axles will fit a 9 inch just for you’re reference.

And yes, a clogged vent line may cause an over pressure in the diff at highway speeds causing a seal to blow out.
 

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Westmus, here are additional pics ....
The last pic show the vent without a hose mounted and it looks dry. Check that it's not clogged by dirt. Driving generates heat inside the axle and if the overpressue can't get out of a vent, it will try to push oil out through any weak spot.
 

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In addition to the road source of dirt for clogging small round orifices, there are several kinds of wasps and other insects that like to take advantage of small round spaces as breeding locations. They fill it in with mud and lay their egg inside it. It's best to never expect an exposed, small round tunnel to remain open. Another good reason to always have a vent hose connected to a vent that might be attractive to an insect.
 

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If you have a limited slip differential, friction modifier additive (still available from Ford over the parts counter) may quiet down the differential if the axle bearings are otherwise OK. From your pictures, there appears to be a zerk type grease fitting on the passenger side of the axle. What it that for?
 

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Discussion Starter · #9 ·
If you have a limited slip differential, friction modifier additive (still available from Ford over the parts counter) may quiet down the differential if the axle bearings are otherwise OK. From your pictures, there appears to be a zerk type grease fitting on the passenger side of the axle. What it that for?
About the LSD: Yes it is an LSD. Yes I put modifier into it a year ago, but it also looks like all of that is gone already.

About the Zerk: Honestly, I have no idea! :grin2: I have not had a chance yet to make myself familiar with the rear axle. I have a feeling that this will change now.
 

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Discussion Starter · #10 ·
It’s a 9 inch...
Yes, it seems like I'm going to pull the whole thing and rebuild it. It's my first time, so I will trust in Joseph Palazzolo's "Ford Differentials: How to Rebuild the 8.8 and 9 Inch" to guide me through the process. I just hoped to procrastinate a bit longer...
 

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Yes, it seems like I'm going to pull the whole thing and rebuild it. It's my first time, so I will trust in Joseph Palazzolo's "Ford Differentials: How to Rebuild the 8.8 and 9 Inch" to guide me through the process. I just hoped to procrastinate a bit longer...
If you’re doing it yourself you’ll need a few special tools such as a dial indicator with magnetic base. A big *** Johnson bar to remove the pinion nut and to reinstall it while crushing the crush sleeve. You’ll also need a good torque wrench and a hydraulic press helps a lot for removing and pressing on the bearings. A large bench vice well anchored is advisable too. The axle bearings are fun to remove especially the wedding band (bearing retainer) I usually carefully drill them almost all the way through with a 3/8 bit then use a chisel and big hammer to split it. Then it comes off easy. I would buy 2 crush sleeves just in case you mess the first one up trying to get the pre load. You’ll also need a small inch pound torque wrench to check the rotational torque on the pinion after it’s in the pinion support and the sleeve has been crushed, not the main case. They’re not hard to do if you have the right tools. Also, unless you have a really good air compressor I would back off the pinion nut while the center section is still in the car with the rear wheels on the ground if possible. Just be careful as when you try to turn that nut, the car will want to move and if it’s not secured you’ll be in trouble if you’re under it. Any questions, fire away....lol
 
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Pretty sure all big block cars came std. with 31 Spline Axles, so my guess is this rear axle assembly is not original to this car … that is if it is an original 428 car.

Not a 31 Spline axle expert, so I'm not sure if they will interchange in an original 28 spline housing or not (along with the required 31 spline center section).

I've heard the reference to big bearing & small bearing axles & housings and am pretty sure all 28 spline are small bearing, but whether 31 spline was available in both big & small or just big I don't know.

I guess my point is this … if it is a factory 428 car, I'd think you would want to do a little research and try and make it as correct as possible when going though the effort & cost of rebuilding it and re-installing. It may cost more money to do it right, but then it is done and it should add value to the car as well.

Doug
 

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28 and 31 spline will fit in the housing, (As long as the side gears in the carrier are matching to the axle) the bearing surface on the shaft is the same diameter for both. The difference is in the housing where the bearing sits. Most big bearing housings were in Trucks, Lincolns and the odd station wagon. Other than that they were all small bearing. The O/P has 28 spline axles as is evident in the one picture which shows the outer axle end. If it were 31 spline there would be a large drilled hole in the center with two smaller holes opposite each other on the outside of center.

Yes, one would expect an original factory 428 car to have 31 spline axles but with Ford back then....who knows.
 

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Not sure where 428 car came from, that is not a nodular rear and the exhaust strap welded to the passenger side makes me think otherwise. That and the fill plug in the diff housing makes me think it’s from a 67.....
Yes the housings will fit either 31 spline third members or 28 as well as the axles, the axle seal is different fir the 31 spline version. If it is, kudos for having one.

As to the original poster, I’d start by just pulling the axles and checking the bearings, since you have a leak on just the one side that bearing may be bad. Once the axles are out turn the yoke by hand and feel if there is any roughness as it rotates. If so, then remove the diff, it’s heavy, be careful. It’s not a complicated unit, but it will require some specialized tools to set it up properly if you decide to rebuild it.
 

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Not sure where 428 car came from, that is not a nodular rear and the exhaust strap welded to the passenger side makes me think otherwise. That and the fill plug in the diff housing makes me think it’s from a 67.....
Yes the housings will fit either 31 spline third members or 28 as well as the axles, the axle seal is different fir the 31 spline version. If it is, kudos for having one.
from his signature:

__________________
70 Fastback Mach1 428CJ

Doug
 

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Not sure where 428 car came from, that is not a nodular rear and the exhaust strap welded to the passenger side makes me think otherwise. That and the fill plug in the diff housing makes me think it’s from a 67.....
Yes the housings will fit either 31 spline third members or 28 as well as the axles, the axle seal is different fir the 31 spline version. If it is, kudos for having one.

As to the original poster, I’d start by just pulling the axles and checking the bearings, since you have a leak on just the one side that bearing may be bad. Once the axles are out turn the yoke by hand and feel if there is any roughness as it rotates. If so, then remove the diff, it’s heavy, be careful. It’s not a complicated unit, but it will require some specialized tools to set it up properly if you decide to rebuild it.
So is it a real 428 car???
 

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The axles and bearings can be a bugger... Turn a brake drum backwards with a lug nut or 2 and use it as a slide hammer while the diff is in the car. Worked great after I said many bad words trying it EVERY other way!

BTW 9 inch is easily identified by the bottom carrier bolt being inaccessible with a socket, because it's shrouded by the pinion; it must be loosened with a box wrench.
 

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Discussion Starter · #20 ·
If you’re doing it yourself you’ll need a few special tools such as a dial indicator with magnetic base... Any questions, fire away....lol
Thanks Skidoodave! Except for the hydraulic press, everything is in place and ready for shenanigans. I'd hoped that one of the workshops around here would have a press I can use. I will definitely have questions once the third member fell on my foot. :smile2:
 
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