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Sure grip rear end questions

74Runneer

74Runneer

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I've got a 74 road runner with a 318 and a 3.23 open diff rear end. I'm eventually gonna swap in a 408 stroker but I want to upgrade the suspension and rear first. I've been looking all over but can't find the answer to my basic question:

If I buy a 8 3/4 open diff rear end, say with 3.55 gears, can I install some sort of limited slip (auburn or eaton) to it and convert it to a "sure grip"? Or do I have to have a third member that had factory sure grip?

Being a 1974, I've heard it's difficult to find a sure grip, or even an 8 3/4 for that matter. What's the best way to piece together a limited slip rear end that can handle 400-500 ft lb of torque?
 
I like the Truetrac they make the 30 and the 33 spline for the 8 3/4
 
What rear end is in the car now?
 
74Runneer said:
$1400 for a third member? Ouch. Is that what I should expect to spend?
Click to expand...
To just have a sure grip unit added to your (likely 741 casting?) will run around 800-1000 or more depending on the shop and that's re-using all your bearings, gears, parts & pieces etc...you can definitely put a sure grip in a center that didn't have one originally. Doing it yourself will save you some cash but you need to know what you're doing
 
It's a 8 1/4 right now (don't know the casting), so I'll have to get a new housing too.
 
74Runneer said:
It's a 8 1/4 right now (don't know the casting), so I'll have to get a new housing too.
Click to expand...
In that case you can:

(1) start hunting the junkyards/for sale sections/etc for good rebuildable cores, buy parts as you can afford, then have a good shop/person do the final build-up, (or go for it and learn to do it yourself)

(2)Buy (lot of $$$ all at once) a complete pre-built rear...strange, dr diff, etc

Personally for me the journey is a lot of the fun, so I like to hunt and gather parts as budget permits, then build it..not a ton cheaper in the long run but, the experience is worth more than $ sometimes in my opinion... you could just get a sure grip for that 8-1/4 but how long would it live with the stout stroker, is the concern..
 
Yeah I'm gonna redo the front suspension over the winter so maybe by the following winter I'll have gathered all the necessary parts for the rear.

And if I ended up blowing up my current 8 1/4, are the failures catastrophic or are they pretty well contained? I don't want to risk putting a chunk of metal through the sheet metel.
 
So how would I go about installing an auburn or Eaton sure grip in my 8 1/4? Is this something that should be handled by a specialist? Or could a mechanically inclined person do it? Are there a lot of special tools?
 
Something you might try is doing some suspension bias mods. In the 60's early 70's, very few gave much thought to limited slip rears and since they were an added cost, most dealers didn't order them for stock on their lots so there were not that many to be had from the wrecking yards etc. as they started showing up in them. Also, most guys I knew bought the cars used so the majority had hot rods with no limited slip rear end. Well, I found out if you crank up the left front torsion bar a bit, it would put more pressure on the right rear wheel. Keep in mind that when you hammer the throttle, the rear end tries to torque over counterclockwise which makes the right wheel try to lift. It's not much but it's opposite of what you want and it's enough to make the left wheel plant better and the right side not. That's why the right wheel is pretty much always the one that spins. You can also add a leaf or two to make it even better. Another thing you can do that's even easier than adding leafs is to install an air shock to the right side. Crank up the left front bar a 1/2 a round or so, put 10-15 psi in the air shock and give it a try. It might surprise you....it did me.
 
Cranky said:
Something you might try is doing some suspension bias mods. In the 60's early 70's, very few gave much thought to limited slip rears and since they were an added cost, most dealers didn't order them for stock on their lots so there were not that many to be had from the wrecking yards etc. as they started showing up in them. Also, most guys I knew bought the cars used so the majority had hot rods with no limited slip rear end. Well, I found out if you crank up the left front torsion bar a bit, it would put more pressure on the right rear wheel. Keep in mind that when you hammer the throttle, the rear end tries to torque over counterclockwise which makes the right wheel try to lift. It's not much but it's opposite of what you want and it's enough to make the left wheel plant better and the right side not. That's why the right wheel is pretty much always the one that spins. You can also add a leaf or two to make it even better. Another thing you can do that's even easier than adding leafs is to install an air shock to the right side. Crank up the left front bar a 1/2 a round or so, put 10-15 psi in the air shock and give it a try. It might surprise you....it did me.
Click to expand...
I had a 65 Chevelle that ran hard but was not positraction. They have coil springs in the back and I could buy " Air Lift" brand air bags that fit inside the coils. Didn't even have to dis assemble the coils. We used them to raise the rear of the car up , and found that by adjusting the air pressure in them , each individually you could pre load the right side tire to make it lay rubber equally on both sides--same as posi traction. They were not expensive. There was a kit to install them on a leaf spring car. Don't know if they are still available......................MO
 
I've been down this road myself. Here's what I learned...

1. There are not a lot of 71-72 8.75 rears out there, and most of the time when you find one the axles are gone, so it's essentially worthless. I've found two complete units for sale over the past five years, so the odds aren't good that you'll find one.

2. A much better option is to find one from a 1970 or 1971 C body. This is a direct fit for the 71-74 B bodies. Seems like every junkyard I go to has one or two old C bodies sitting somewhere, and there always seems to be a few ones getting parted out on Craigslist. This one is in your area: https://minneapolis.craigslist.org/dak/cto/d/chrysler-parting-out-update/6230160801.html

3. If you can't find a 70-71 C unit, you can use an E body unit, which is what I ended up doing. You will need to cut the spring perches off and have new ones welded on .75" inward, but most every E body made had an 8.75 on it so there are a LOT of them floating around out there at cheap prices.

4. For the center, wait and buy one that meets your needs. Look for a 3.55 Sure Grip that's already pulled and ready to ship. You can buy an open unit for anywhere from $100-$300, but by the time you spend the money for a SG unit, and the gears you want, and then for a tech to build it, you'll be in for twice what it would cost just to buy the right unit from the start.
 
I agree with both Cranky and Bru.

I did the air shock trick on my first 66 Coronet and it was a major improvement.

Didn't really leave two black marks, but the power had a much better chance of getting to the wheel with traction.

E body 8 3/4 axles are usually in the 100-200 range and might even have a 489 center but probably with 3.23 or 2.76 open gears.

A sure grip center with a 3.23 for 300-400 bucks is not unheard of, but a 3.55 is harder to find and more pricey.

A new SG/limited slip unit is way more pricey and requires removing the ring and pinion, then reinstalling with some pretty advanced setup.

The ring and pinion are matched to the case casting (741/742/489, etc) but the SG unit is not.

A new R&P can be had for $200.
 
Last edited:
Another option for your 74 is to find a 9.25 out of a 74-79 B. They are good diffs and they make gears and Suregrips for them. Can be a lot less than a 8.75 in the end.
 
74Runneer said:
I think I want to just get the rear end and not have to tweak the suspension.

Bruzilla,

It looks like I would have to cut the perches off that c body and move them in, right?
Click to expand...

Yes you will. You might also want to check the pinion angle they provide as well. The nice thing is the overall width is the same. The width of the E body rear is about 1.5" less, which wasn't an issue on my 74 Roadrunner until I added traction bars and had to cut some material off the bars' rear flanges.

This is also a good time to replace the leaf springs as well.
 
Yeah, my springs are sagging pretty bad right now. Although I kind of like the height.

What's a fair price for an open 8.75 rear end from a c body? I'm waiting to hear what gears are in it and what casting the diff case is.
 
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