318 cars have 8 1/4 diffs?

[F body Deconstructor Jim]

Probably is a 7.25.
The 88 Diplomat 318 I just parted had a 7.25.
One of those confusing 7.25's that looks like a 8.25 until you get closer to it.

 

[Jack Meoff]

I'll check tomorrow but I'm pretty sure my Fifth has a 7.25

 

[MBDale]

Got the differential out of the car and it's at a rebuild shop. 3:55's going in. It's got factory posi! Trans (727) getting a rebuild too.

 

[BudW]

Pretty much (about 95%) all F & M bodies have 7¼” differentials.

That said, all Wagons as well as towing package cars, most Police and most (but not all) manual transmissions got the 8¼” differential.

BudW

 

[kkritsilas]

All 3 of my J bodies (1980 Cordoba, 1981 Mirada CMX, and 1982 Mirada Base) have 8.25" differentials. All are V8s, the first two 218-4BBL/A999, the Mirada base is a 318-2BBL/A998. I would guess that they have the 8.25" rear either due to their being early 1980s cars, or because they used the heavier duty (A999/A998) versions of the A904 transmission, and the 8.25" differentials were packaged up with the heavier duty transmissions.

 

[dodger roger]

I've never seen a V-8 F/M/J body with a 7 1/4", that's not to say there weren't any though. On the other side of it, my dad's S6/3-speed manual '74 Gold Duster has an 8 1/4" not the expected 7 1/4", go figure.

My 80 Cordoba 318 2 barrel A727 automatic 7 1/4 rear differential with 2:45 gears

 

[7T8 Custom]

My 78 318 904A 2-barrel Aspen was originally a 7 1/4. Now an 8 1/4 after exploding the spider gears twice.

 

[Bruceynz]

My 83 Cordoba with a 318 had a 7.25" now has 8.25" and am going to put a LSD into it, the LSD carrier has arrived in NZ but stuck in customs, gov will want some tax on it, sigh!

 

[XfbodyX]

My T top 4 speed superpak cars one a 77, one a 78 both came/have with the 7 1/4. Ive often wondered why more of the rear axle/rear gear wasnt on our fender tag codes.

 

[BudW]

Ive often wondered why more of the rear axle/rear gear wasnt on our fender tag codes.

A very good question (which I do not have an answer for).

I suspect the fender tag was made for the body people on the assembly line – but even that doesn’t make sense for all departments within the assembly line.

The build sheet plus the fender tag is what was used for building the car, with – but most FMJ’s did not get a chance to keep their build sheets.

There is an example of a built sheet (this case under the back seat.

BudW

 

[XfbodyX]

Ive the sheet to one of them and the older mopars I own but do wish the had included the axle info on our F cars on the tag codes.

 

[Mr C]

I've owned many V8 M bodies over the years...all were 7.25 diffs. I agree with BudW's assessment that 95% of F and Ms had the 7.25...checks with my experience.

 

[Aspen500]

Looks like I stand corrected! lol Wouldn't think a 7.25 would live behind a V-8. My car's original 7.25 broke the spider gears once and that was behind an E24 S6 (CA emission 90 hp) and 4-speed O/D trans and I did NOT abuse it at all. Either way, 7.25 just seems too small for anything other than a 6 cyl in my mind.

 

[BudW]

First off, I have not worked on enough J-bodies to know what percentage are 7¼” vs. 8¼” – but I suspect to be pretty lopsided (like 90%+ are 7¼”) – but not enough experience to say for sure.

The 7¼” is a pretty weak unit – but does seem to hold up well to 318 or 360 2-bbl automatics ok. Any manual transmission will exhibit more driveline shock – even when being gentile.

You install a 4-bbl along with any additional performance parts – the 7¼” will shatter and leave a trail of broken parts behind you.
Non-performance parts and only a 4-bbl and headers with an automatic, you may be fine – but wouldn’t hurt to get a spare 7¼” and slip under your workbench in your garage.

An 8¼” can take a lot of punishment, for the most part.
Get a torquey big block or drive hard with a manual, getting an 8¾” or Dana 60 (either from a ’65-70 B-body) or getting a 9¼” cut down to fit, would be a very good idea.

I do agree with Aspen500

Wouldn't think a 7.25 would live behind a V-8.

But they do fairly well, for the most part.
BudW

 

[volare 77]

I have owned 5 volare aspen cars at different times over many years. 4 of the 5 have been 8 1/4. Two with 318 and manual trans were 8 1/4. The two 76 volare rr with 318 and auto had a 7 1/4 in one and 8 1/4 in the other. My 77 with a 360 has a 8 1/4. All the cars had the original rear axles. The 7 1/4 axle didn`t do well while doing a burnout going around a turn. The spider gears on a open rear usually went first.

 

[BudW]

I have owned 5 volare aspen cars at different times over many years. 4 of the 5 have been 8 1/4.

Great!

I’ll go back to what I originally said:

Pretty much (about 95%) all F & M bodies have 7¼” differentials.

That said, all Wagons as well as towing package cars, most Police and most (but not all) manual transmissions got the 8¼” differential.

BudW

If vehicle had the towing package, it got the 8¼” - as well as most (but not all) manual transmission cars.
I would guess one of your '76's and your '77 with 360 had trailer towing packages.
It looks like you picked some winners!


I have heard of supply shortages that may have caused a different differential to be installed.
I have also heard that once the M-body was axed, in ’89, Chrysler also immediately quit making the 7¼” differential (for M-body and Dakota usage) – so (as rumored) all M-bodies made after the word got out about the body line was cut, all used 8¼” differentials.
I can’t prove or disprove either rumor (other than what I said, in above quote).
BudW

 

[volare 77]

Don`t know about trailer towing or not. All were either Volare Road Runners or Aspen R/T`s. Maybe that had something to do with it. My 77 has heavy duty leaf springs.

 

[Aspen500]

Maybe it was just the luck of the draw or other equipment aside from the 318 option(?) As I mentioned, my dad's '74 Gold Duster S6/3-speed manual has an 8.25" axle for whatever reason (the car's options are few, AM/FM and,,,,,,,,,,,,,that's pretty much it, no power anything), yet some FMJ with a V-8 have only a 7.25".

 

[XfbodyX]

To throw a monkey wrench in the speculation here, my 77 t top 4 speed 7.25 R/T has the towing pkg, hitch and all, I was fortunate enough to buy it from the original owners family which came with some pics of its early years. Ive not looked at the FT in years but will look today at the codes. But the pics I do have never shows anything hooked to it and the last contact with that family was over ten years ago via email.

I think ive also a new kit in the bag of the "Tow Package" hitch which by the standards of today looks to be of lesser strength and quality then on my John Deere garden tractor. IIRC two of the mounting points are in the bumper itself and two in the cross bar of the hydro bumper shocks. Id hate to speculate how low the "tow" rating was on our cars.

Does anyone have any old literature of fnj car hooked towing anything? I never looked but I dont think I had seen any in the mix?

 

[Aspen500]

How about another monkey wrench? Who's to say the cars actually got the axle that was SUPPOSED to be installed? We all know what automotive build quality was like in the 1970's..................You never know, the line workers might have just bolted in whatever was close at hand. For towing, the rear axle size has more to do with tongue weight than actual trailer weight. It's mainly the axle shafts and axle shaft bearings. That's why HD trucks have full floater axles with HUGE hub bearings. It's to carry the weight of a full load. I'm just thinking out loud on most of this stuff, take it for what it's worth!