Well, I was taking a cruise from Houston to Corpus Christi Thursday evening for a movie shoot when a rear wheel bearing gave out, and my axle came sliding out of the rear end at 65 mph. Thankfully, it didn’t come all the way off, and I was able to safely get the car to the side of the road.
I’m in a situation where I need to replace the driver side axle, the wheel bearings and seals on both sides, pull open the housing to inspect and possibly replace the gears in the differential, and since there’s gear oil all over the driver side brake shoes, I might as well replace my shoes and wheel cylinders on both sides while I’m doing all of this work.
Since I’m doing all of this work anyway, I’m debating whether or not to swap in a locking differential with a steeper gear ratio, maybe 3:55?
If I’m spending the money for a new center section, what are the pros and cons of just swapping out the 8" rear for a 9"? Does it mount the same? Can I use the same brakes or does the 9" take different drums brakes? Does anyone sell complete 9" assemblies ready to bolt in?
If I was to go the cheapest route and leave it stock, what’s the best supplier for just a replacement axle?
Traction and maybe wheelhop is what breaks them. If you’re going to run street tires with an automatic and less than 300RWHP, you’ll be fine with the 8”. The 8” don’t last long at the drag strip where you can hook hard and suddenly.
But you can spend a quick $3,000 on a drop in aftermarket 9” setup. Don’t do it if you don’t need to.
pS- 3.55 rear gear is about my limit with a non overdrive. If you do 3 hour drives like you just did with more than that, you’ll hate your car by the end of it.
I have an AOD, so it helps with the highway. I just figured if I ended up having to rebuild the rear end anyway, that maybe I could do something more exciting than the 2.79 gears I have in there. Maybe I should just do 3.25?
The 8" is plenty strong. You will get away cheaper just getting the new bearings and seals done. The ring and pinion are likely just fine.
Couple cans of brake cleaner will likely cure the lube soaked brake shoes, change out the fluid and put it all back together. Like was said earlier, you can easily get in over your head switching to a 9" and needing all new stuff. Nothing will interchange with the 8" you already have.
And…, while the 9" rear housing from a Cougar WILL bolt right in, no one else mentioned that the driveshaft also needs shortening when swapping to a 9".
Actually what is said below is somewhat incorrect. The 9" rear axle uses the same brakes, wheel bearings, and the same axles if you choose to make it a 28 spline rear.
As for the driveshaft, the 9" rear axle will accept the same driveshaft if you get the right universal joint for the swap. In my case I used the 9" 28 spline housing and differential from a '69 Cougar, along with the brakes, driveshaft etc. that were existing. The universal joint needed to adapt the rear of my small block '68 driveshaft was a '69 Camaro application with 307 engine and automatic that made the interchange a bolt in with the existing drive shaft. The small block Cougar U joint doesn’t fit the 9" yolk.
I agree the 8" rear is plenty strong so long as you are not going to do any hard launches with sticky tires. If you plan on any of that, the 28 spline axles and the 8" differential won’t last long.
Shorting the drive can cost up too $300 pending on the drive line the factory drive line are pressed in. I just had one shortened to connect to my 4 speed.
I also changed to a 9 inch rear on my 1968 XR7. It was the most cost effective way for me to get a traction lock.
I already had a 9 inch from a 1969 Cougar that I had parted out. Also had a set of 31 splined axles. A $90 pick-a-part 9 inch 3L50 trac lock from a Ford Bronco. *Note - Most light truck variety 9 inch pumpkins use 31 splined axles and have 1330 pinion yokes.
Left the brake backing plates connected to the hard lines, when I removed the old 8 inch rear. The 1967-68,1-3/4 inch rear brake shoes are still adequate. Could have upgraded to 1969-70, 2 inch rear backing plates and shoes, but chose not to.
Swapped in the 9 inch housing with 3rd member installed. Reassembled the axles with new bearings and seals.
Because I also had changed to a T5 transmission, I knew that I would need a different length drive shaft even, with 8 inch rear.
Lucky for me, a FMX drive shaft was the proper length. Swapping to a C-4 transmission yoke, and a combination U joint for the rear (1310/1330) and it all went together with mostly used, and scrounged parts.
If you are interested, I have a original 8" center section with limited slip posi 3:55 gears. In ready to bolt in condition. I would sell for $500.0 + shipping
That sounds like a deal. I’ll PM you for more details.
This is probably a dumb question, but I haven’t pulled the passenger side axle yet to look. Does the passenger side alrady have splines even if you have an open differential or will I need to change that axle out as well if I go to traction lock?
I agree in sticking with the 8" for your application, like was mentioned you can not just dump a 9" center section in the 8" housing. Its all or nothing with the exception to brakes. A 9" would be a good upgrade if you planned on a harder hitting power-train and lead foot but otherwise I think your good with the 8. A limted slip and proper gears could be a good upgrade for ya however.
Junk yard parts are a nice option if you have a good selection in your area. For me its quite hard to find a good 9" in anything anymore. They have mostly been picked clean or want way too much for junk. Since the 9" has not been in anything in quite some time they are starting to become limited but your results may vary.
Always measure to be sure, but often switching out the differential yoke from the typical Mustang/Cougar style to the shorter pickup style will provide the necessary clearance without modifying the driveshaft.
Again, switching to a 9" no longer makes sense as it did in the 90’s. This unit will support 500 HP and fits right into your original housing. It uses less horsepower to operate than a 9" and he can build them to accept 31 spline axles.
9 in ring gears are stronger than 8" for sure. In 1990 - 2019! But the weak link is the 28 spline axles ,which are the same in both axles. Ive busted plenty of them!! But with that being said, I’ve changed from 8" to 9" in the last 68 that I built. And I dont see it as an asset at all in a street car.
