318 cars have 8 1/4 diffs?

[Oldiron440]

Why do you think it has to do with specs? Put in what they had...!

 

[BudW]

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 brings up another whole issue.

The 8¼” (and 9¼”) differentials. The wheel bearing race is on the axle shaft itself – which presents a big problem.
I cannot tell you how many axle shafts I seen wore into, by the wheel bearings.
The offset wheel bearing only recently came out – which helps to save a damaged axle shaft – but beforehand I saw axle after axle (and wheel bearing after wheel bearing) replaced on both the 8¼” and 9¼” differentials (B, C, F, J, M and R bodies, as well as pickups and vans - a lot more-so on the heaver vehicles). Even replaced a few axle shafts for the Tulsa police department F & M police cars.

Also, some of the Tulsa police dept. cars had 7¼” in them – which didn’t last very long (Tulsa bought on the very low side of the bids). I had even replaced a couple of 8¼’s on the cruisers, but because of other reasons (one hit a curb so hard I couldn’t get the bent axle pushed in, to remove the C-clip) and other one ran over something which bent the propeller shaft and bent the differential into a boomerang.


A 9¼“ is much stronger than an 8¼” is – but will not/cannot carry any more weight than the 8¼” does – because of the wheel bearings.
That said, I wouldn’t worry much about trailer towing capacity on an FMJ, as far as the differential is concerned. The brakes and transmission – I would be more worried about. The A999 is the only one that has adequate clutch material to tow a lot of weight with (besides the pickup transmissions).
If you have a car with 2.2 gears in the differential, you will not be to move a lot of weight, to begin with (the car will just sit there and won't move).
BudW

 

[AJ/FormS]

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.

I have one of those reprobate imposters,lol

 

[AJ/FormS]

That brings up another whole issue.

The 8¼” (and 9¼”) differentials. The wheel bearing race is on the axle shaft itself – which presents a big problem.
I cannot tell you how many axle shafts I seen wore into, by the wheel bearings.
The offset wheel bearing only recently came out – which helps to save a damaged axle shaft – but beforehand I saw axle after axle (and wheel bearing after wheel bearing) replaced on both the 8¼” and 9¼” differentials (B, C, F, J, M and R bodies, as well as pickups and vans - a lot more-so on the heaver vehicles). Even replaced a few axle shafts for the Tulsa police department F & M police cars.

Also, some of the Tulsa police dept. cars had 7¼” in them – which didn’t last very long (Tulsa bought on the very low side of the bids). I had even replaced a couple of 8¼’s on the cruisers, but because of other reasons (one hit a curb so hard I couldn’t get the bent axle pushed in, to remove the C-clip) and other one ran over something which bent the propeller shaft and bent the differential into a boomerang.


A 9¼“ is much stronger than an 8¼” is – but will not/cannot carry any more weight than the 8¼” does – because of the wheel bearings.
That said, I wouldn’t worry much about trailer towing capacity on an FMJ, as far as the differential is concerned. The brakes and transmission – I would be more worried about. The A999 is the only one that has adequate clutch material to tow a lot of weight with (besides the pickup transmissions).
If you have a car with 2.2 gears in the differential, you will not be to move a lot of weight, to begin with (the car will just sit there and won't move).
BudW

Kindof like my LaCross stuck in two inches of snow with the traction control engaged. The more she stepped on the gas, the dumber my wife felt. I just reached in and turned off the switch, and she backed right outta there,lol. She never got stuck with that car again.And we owned it for 10 years.
Then she wrote it off. She's good at writing off cars and not getting hurt.

But I must say, if you have a choice between 3.21s and anything bigger, go bigger. Teeners come alive with lotsa gear. I wouldn't ever go less than 3.55s with 15" tires, so long as the engine remains stock. I've run them with 2.45s to 4.88s and the 4.30s were the best.......but I had an overdrive. Hyup a 318 with 4.30s can really get moving. And I wouldn't be scared to keep the 7.25, so long as it had a LSD in it. Or like N50-15s. Hyup I did that too, once, on a 69 Barracuda 225/904/7.25. The little slanty had no problem not peg-legging even just one of those,lol.
I crashed that car on my wedding night, being in too much of a hurry,getting back to the party. Not enough to write it off, but too much to carry on. After a short break, we hopped into a taxi, and carried on. That was April-1977, and that 69 had been mint up to that day,lol. Well it was only7.5 years old, daymn.

 

[slant6billy]

Last week Gator and I found a 8 3/4 out of and e body (741 open) and there was an 81/4 with isoclamps / leaf springs still on it. When we went back, the 8 1/4 also had a sway bar (cop rear most likely). 44 1/2" center to center on the 8 1/4 perch

Going to try to buy as a package deal. My 77 has a 7 1/4 (4 speed / super 6 ) and could really use the 8 1/4 with the anti-sway. My plans for the 77 is ralley track ( corners and curves)

 

[BudW]

’65-74 C and E-body both share the same perch distance of 46”.
’75-78 C-body and ’71-81 B/R-body went to 47.3” perches.

Note: 8¾” E-bodies (and ’69 C-body) are 56.437” flange to flange (backing plate to backing plate).
‘65-74 8¾” C-bodies have other flange to flange measurements of: 56.750”, 57.875” and 59.437”.

The 8¾” might be from a C-body, but there will be a lot more usage for or sales opportunity for an E-body.


That particular 8¼” is an FMJ unit.
A big plus if it has the rear sway bar and its brackets. It might also have 11” brakes and 5-leaf springs – both of which are worth grabbing.
Bud

 

[Mr C]

I will say this. The 7.25 lives just fine behind a stock 80's smogster V8 making...what 165 HP tops? I've never had a 7.25 fail on me on my stock daily drivers.

 

[Mopar&vettedude]

That exactly just happened to me today. I had dropped off the rearend at the shop only for them to call me and tell me its a 7.25. Kind of bummed out now....

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.

 

[BudW]

I don’t think even the factory kept track of production numbers of differentials, but my experience and history, I would say that 95% of all FMJ’s have 7¼” differentials in them, and that number might be on the low side.

Most station wagons (not all) and most police/taxi’s (not all) came with 8¼”. Some 4-speeds also came with 8¼” – but most did not. A person can special order a car with a towing package and it would also come with an 8¼” – but not a lot of people special ordered FMJ’s.

Two tips I have when looking at FMJ differentials.
First all 8¼” ('63 to 96) have an oval inspection cover, using 10-bolts.

And straight (3”) axle tubes


7¼” differentials changed a few things between the years. The differential covers are either (roughly) diamond shaped or (roughly) octagon shaped. The covers either have 9-bolts (earlier) or 10-bolts (later).

'76 to 81

'82-96
The axle tubes are either straight (2½” diameter) tubes (earlier) or 3” tubes that taper down to 2½” just before it goes into the center section (later).

Believe me, I’ve seen a lot of people said they were sold an 8¼” but what they walked away with was an 7¼”, over the years.
BudW

 

[Mopar&vettedude]

Thats just it, i for the longest time thought I had a 8 1/4 rear. The diff cover looked dead on to the 8 1/4, but for some reason the ring gear ended up being 7 1/4. The guy at the shop thought the same until he popped the cover off.

 

[volare 77]

Out of the 6 f bodies I have owned 5 of them were 8 1/4. All were 318 cars except one 360. 3 were auto trans and 3 were OD trans

 

[AJ/FormS]

Three of the Five that I have owned were 7.25s; two were /6-autos, and one was a 318 auto 84 FA. The missing two were 318 auto wagons and I never looked,lol.

 

[Magnum Aspen]

My 77 wagon came from the factory with a 318 and 8 1/4. Accord8ng to the broad cast sheet and pictures from this thread.