7 1/4 why weak?

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barbee6043

barbee6043

barbee 6043
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I know some people will swear by the 7 1 /4, maybe all the ones I've had were already woreout when I got them!? just wonder if anyone can give some info why it is weak? I got a couple in early A slant cars i'm working on.
I 've had many B bodies and few A bodies with the tried and true 8 3/4. can anyone give some reasons why the 8 1/4 is a good rearend, other than obviously a little bigger!????? thanks for any input.....
 
I have flat out beat on plenty of 8 1/4 rear ends in B bodies and never broke one. Something else always broke first. I have never used one behind a race engine. I have beat on a few 7 1/4s behind 318 and 360s and never had one fail either. I hear the spider gears are the weak link but I don't know first hand
 
I bought my first Dart, from a school teacher, with 56,000 miles on it. Put another 50,000 miles on it before the spider gear failed in the 7 1/4. This was a mild 273(4 barrel, small cam) that I beat on constantly.
 
I broke 3 7 1/4 axles in 4 years in my old 68 Barracuda with a 318 with 9.2 compression, stock 340 cam, 600 vac secondary and dual exhaust.

Basically a 318 with a 340 cam and 4 bbl and dual exhaust. It can't handle much over a stock 318.

that's why they are weak.

After I went to an 8 3/4" axle, I never broke another one again for over 200 k miles on that same car....
 
I had several 7 1/4 in A bodies, that had whin. I never actually broke one, but then again I didn't beat on them. granted they probably had many ( maybe several huindred miles~!???). it is unusual to find a Mopar mechanical part from back in the 60's that was weak.
back 10 years ago, an A body 8 3/4 was not scarce or expensive! you know the 8 3/4 was strong as it was standard on auto hemi cars! I've had a couple of A cars with 8 1/4 and had no trouble with those but then again I ain't a racer! can only afford to build it once!!! LOL
 
The old style sure grip 7 1/4's wernt too bad @ all. (the clutch Style) But as a you guys allready know there RARE & parts for the SG are even harder to find. but they are tough. (My car came with one from the factory,i also know how rare they are & how i like to drive so i sold it while it was in good working order & ponyed up for a 8 3/4) the weak link is for sure the under sized spider gears. in the open & cone style SG centers.
 
Simple. It's weak because it is small.
 
i had a stout 340 in my '68 dart, it had a 4 speed. heavy duty clutch.

one time i did a launch and it broke the yoke right off. was an 8 1/4

it would be the same deal with a trans brake or very high stall converter with foot braking. the sudden impact is what breaks stuff.

if you just have a stock auto and stock converter you will never hurt it. depending on the engine specs, i bet a u-joint would fail first.
 
this 65 cuda is original 273 2 bbl car, auto, with 7 1/4. so is it any stronger than a 7 1/4 say in a slant car or later car???? I am picking up some 8 1/4 and 1 8 3/4 out of a 65 b body, to have when needed. just wondering bout the 7 1/4.
small? mighty mouse was small!?? LOL
 
There are only 2 versions of the 7-1/4. The open differential and one with a sure grip unit. The sure grip unit was beefier and will take more abuse than the open unit but in the end it's still a light duty built rearend only suited for small low hp engines.

The next step up is the 8-1/4 which is built beefier than the 7-1/4 and it'll take quite a bit of abuse. And step up again to the even beefier 8-3/4 that'll take a lot of beating on

If you lay all the insides of a 7-1/4 side by side next to a 8-3/4 parts you'd think the 8-3/4 was built for a truck and the 7-1/4 for a Plymouth Cricket
 
Now I'm lookin for one of these youngsters to ask "What's a Plymouth Cricket?"
 
I was talking to a guy that put a 440 into a dart, the motor had some work done to it, the car still had the 7 1/4 open rear end in it. He was about to put an 8 3/4 in but wanted to take it for a spin to see how the motor ran, was cruising and gave it some gas, well the rear didnt really like that and decided to completely blow apart. Didnt wreck the motor or trans just stripped everything out inside the differential like it was made of butter lol.
 
RustyRatRod said:
Now I'm lookin for one of these youngsters to ask "What's a Plymouth Cricket?"
Click to expand...

Man, even econocar's were sexy then!

cricket1.jpeg


cricket2.jpeg



Time for an SRT 2.4!
 
I just call the 7 1/4" axle the "Weak-Suck rear end"....


If you can blow it up with a mild 318 4 bbl, it's weak and it sucks....
 
Plymouth Cricket! I remember a dude in HS jammed a 440 into a Dodge Colt.
(He did not back it up with a 7.25)
 
I broke a 3.55 7.25 SG with a 340 and 10 inch wide tires, but it took a neutral drop coasting backwards out of a driveway to do it. From what I could tell after popping the cover off was the pinion broke a bunch of teeth off and the ring gear busted a couple too. Spider gears...no comment as I had seen enough. I replaced it with a 2.76 peg legger (only one I could find over that weekend) and that car was never the same. You can break them, but it takes some dumb driving or some power to do it, or you can just get a weak one.
 
crj1968 said:
Plymouth Cricket! I remember a dude in HS jammed a 440 into a Dodge Colt.
(He did not back it up with a 7.25)
Click to expand...



I remember a guy at Mopar Nats one year with a Blown 426 Hemi in a Plymouth Arrow...

His license number was "SUICIDE"....
 
The weakest part in a 7.25 is the spider gears. Stomping on it hard from a dead stop and while turning, is what kills them. If you take it easy, and dont drive like a hopped up 16yr old, you can make one last.
 
I've got a pile of seven busted 7-1/4 rears behind my Dad's garage from various vehicles I've owned. The spider gears and shafts are broken out of the case on each of them. The most powerful engine that broke them was a 273 2bbl. What kills them is when one wheel is spinning and then grabs. They are too small for even a slant six car if you live where there is ice and snow. I've got to admit that I did break a couple by being a bit aggressive but the others just failed in normal driving. (of course several of them probably had 100,000 miles on 'em by the time I got the cars)
 
Talk about some serious Mopar chest thumping....the 7.25 rear being strong....you guys are killing me. I need some of whatever you folks are on. Guess you folks don't know how to be abusive. Just because you cruise down the road for 200k miles and the rear doesn't grenade doesn't make it strong.
 
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