Mopar 8 3/4 Rear End (A-Body) - Questions & Help....

Greetings My Expert Friends....!!!

Ok, so I found this Mopar 8 3/4 Rear End (A-Body) on craigslist. Will this work on my 1974 Plymouth Duster with a 340/904 set up? I currently have the stock 7.25 Rear Axle and was told that was junk. Keep in mind I'm breaking everything down to the bone and replacing what can be replaced with new parts. Same would go with this 8 3/4 Rear axle housing assembly.

The seller posted this info in the ad :

Mopar (A-Body) 8 3/4 Rear axle housing assembly... 741 Case open 3.23 gears, small bolt pattern....

Measurements:
Drum to Drum 57-1/8 in
Housing Width 52-5/8 in
Perch center to center 43in

So can you guys explain in "stupid terms / easy explanation / 101 Learning" what work would need to be done to make this work on my car? I have no clue and I'm new to all this stuff.

Seller also mention "small bolt pattern"? Also would I need to replace the drive shaft that's in the car right now?

Last, I'm not building a HOT ROD or a 3/4 Mile Racer. Just a Duster with cool stuff on it.

Again Thanks for all your help. Very cool forum....

Alec

Great start!!!

It looks like you have the u-bolts and shock plates, so just remove the old axle and bolt in the 8 3/4" axle...

The 8 3/4" axle is a little longer in the nose and you will have to get the drive shaft shortened to compensate...

Then slap on some new brakes and you're ready to go...

You need to order Big bolt pattern axles with green bearings

There were two different positions for the splitter for the brake lines... Make sure the new axle has the same position as the old one or the brake hose will have to be longer to make the other location if it doesn't match up with your current axle...

krazykuda said:

It looks like you have the u-bolts and shock plates, so just remove the old axle and bolt in the 8 3/4" axle...

The 8 3/4" axle is a little longer in the nose and you will have to get the drive shaft shortened to compensate...

Then slap on some new brakes and you're ready to go...

Click to expand...

Thanks for the help.

Seller also mention "small bolt pattern"? is that 5 on 4 vs 5 on 4 1/2 ????

Mtrhead said:

You need to order Big bolt pattern axles with green bearings

Click to expand...

How much would that cost? Thanks...

Alec

DaWeezzz!!! said:

Thanks for the help.

Seller also mention "small bolt pattern"? is that 5 on 4 vs 5 on 4 1/2 ????

Click to expand...

Yes, 8 3/4 a body axles were only 4" bolt pattern from the factory...

In 73 they switched to 4 1/2" bolt pattern for a bodies...

Price is about right. If you have time you might be able to find BBP stock axles and brakes. (Assuming your 74 has BBP)

Something to look for is crush where the U bolts go over the axel tube, scoring from the knerling of the OEM u bolts not an issue.

It look really clean.

DaWeezzz!!! said:

I'm not building a HOT ROD or a 3/4 Mile Racer. Just a Duster with cool stuff on it.

Click to expand...

For this application;
If the 7.25 has a Limited Slip in it, then
it is NOT junk.
The 7.25OPEN diff. fails when you peg-leg it, as in spin only one tire. This can overheat the cross-pin and then the spider-gears seize to it and then it eventually blows up, usually just leaving you stranded and walking. Another way it fails is by continually slamming power into it with a clutch; but you don't have one of those. Automatics with lo-stalls are easy on the guts.
And a third way is with big tires, a manual trans again, and a high-compression engine; The constant on-throttle/ off-throttle at mid-rpms eventually wears out the sidegears. You can't get big tires on a Duster until you move the springs inboard and; either narrow the axle, or get custom wheels; AND the 74 340 is a low-compression engine; so...........
Before I would buy a bigger rear end, I would pop the cover, inspect the gears and case, and if in good shape I would probably just put a SureGrip into it with fresh EP oil.
However, if the 8.75 comes in cheaper than a SG for the 7.25 (if you don't have one already), then it makes sense to get the 8.75....... but yur still gonna need a SG for that one too.

The only junk 7.25 is a broken one.

Furthermore, IMO,
IT's NO BIG DEAL to run the sbp 5 on 4 for your application. I wouldn't waste my time/money changing it unless I couldn't find the wheels I wanted in that bolt pattern.
The spare tire argument is a crippling story. Carry a plug kit and a 12v compressor and just drive. I haven' carried a spare in IDK, guessing 30 years. Ok I had a flat exactly once, 3 miles from home. My car has 14x7.5" sbps on the front, and 15x10lbps on the rear. My spare tire well carries a few tools and spare parts.
Just my 2cents.

Oh yeah, about that 7.25.... I ran one behind a hi-compression 340 and a 904 for about 3 years in a 65 Valiant wagon; NO SureGrip lots and lots of peg-legging, I mean it couldn't be helped. That was 1975.
The 904 gave up first,lol..................

If you have the big bolt 7.25 you can use the brakes off it and order big bolt axles...u bolts and shock plates and a drive shaft shorten...

The only difference would be the backing plates off the 7-1/4 don't work on the 8-3/4, 4-bolt on a 7-1/4 and 5-bolt on an 8-3/4...

Man there's a lot of questionable info in this thread...

Mtrhead said:

You need to order Big bolt pattern axles with green bearings

Click to expand...

He may not, if he has a /6 drum front car he already has the 5x4 SBP. And the tapered axle bearings will work fine too.

He says he has a 360/904, but if that was the original engine it would also have a 8.25" in the car.

DaWeezzz!!! said:

Thanks for the help.

Seller also mention "small bolt pattern"? is that 5 on 4 vs 5 on 4 1/2 ????

Click to expand...

Yup, SBP is 5x4", BBP is 5x4.5"

What engine was your car originally? Are you positive you have a 7.25"? As I mentioned, if the 360 was original you should have an 8.25".

krazykuda said:

Yes, 8 3/4 a body axles were only 4" bolt pattern from the factory...

In 73 they switched to 4 1/2" bolt pattern for a bodies...

Click to expand...

In '73 all of the front disk cars went 5x4.5". And all the v8's got front disks. But /6 cars still came standard with drums, and the drums stayed SBP. So '73-December '75 a /6 4 wheel drum car was still SBP. January '76 everything went front disk and BBP.

Dana67Dart said:

Price is about right. If you have time you might be able to find BBP stock axles and brakes. (Assuming your 74 has BBP)

Something to look for is crush where the U bolts go over the axel tube, scoring from the knerling of the OEM u bolts not an issue.

It look really clean.

Click to expand...

No such thing as a stock BBP axle for an A-body 8 3/4.

HemiSSDart said:

The only difference would be the backing plates off the 7-1/4 don't work on the 8-3/4, 4-bolt on a 7-1/4 and 5-bolt on an 8-3/4...

Click to expand...

The backing plates from a BBP 7.25 will work just fine on an 8 3/4 with BBP axles. All of the BBP backing plates are interchangeable, doesn't matter if they came from a 7.25", an 8.25", an 8 3/4 or even a Dana 60.

Many of the later BBP 7.25" 10x2.5" drum backing plates have 5 holes (and use 4 on the 7.25"). If for some reason the BBP 7.25" drum backing plate doesn't have the 5th hole already drilled, just drill it. It's literally the only difference.

Well we don't know everything with what he has and what he needs. I just took 10" backing plates off a small bolt pattern 7-1/4 and they have 4 bolts that hold them on, so what I said is correct. If his 7-1/4 is big bolt pattern then you would be correct...

HemiSSDart said:

Well we don't know everything with what he has and what he needs. I just took 10" backing plates off a small bolt pattern 7-1/4 and they have 4 bolts that hold them on, so what I said is correct. If his 7-1/4 is big bolt pattern then you would be correct...

Click to expand...

I agree, that's why I asked. But if it's a SBP 7.25" then the backing plates only having 4 bolts isn't the reason they can't be used, the backing plate offset is. The number of bolt holes is irrelevant if they're 7.25" SBP backing plates.

72bluNblu said:

But if it's a SBP 7.25" then the backing plates only having 4 bolts isn't the reason they can't be used, the backing plate offset is

Click to expand...

That and the OD of the center hole is smaller on the SBP 7 1/4

Dana67Dart said:

That and the OD of the center hole is smaller on the SBP 7 1/4

Click to expand...

Right, there's a lot of reasons the SBP 7.25's can't be used on anything other than SBP 7.25" axles.

But BBP 7.25" backing plates can be used on any BBP axle. Which is why you have to specify what 7.25 you have and not just say 7.25" backing plates won't work on an 8 3/4. BBP 7.25" backing plates work great on a BBP 8 3/4.