Is a 7.25 rear strong enough?

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Charrlie_S said:
certain 8.8 Fords. I think by narrowing the rear by shortening one axle tube and using two short side axle shafts.
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...OR be a weirdo and use two LONG sides to make an extra wide axle to copy Uncle Tony:

1678871102872.jpeg
 
DC Dart said:
In an open diff 7.25 the spiders seem to be the weak link.
What is the weak link in the 7.25 sure grip units? Would it be the ring and pinion or axles?
If someone was teetering on the low HP scale they could possibly gain a little strength with the sure grip.
I have never killed or witnessed the failure of the 7.25 sure grip units as I do not think there were many made.
It is a bummer about the lack of 8 3/4 units around. I am glad I swapped mine out when they were still plentiful. One of my a body 8 3/4 rear ends came from a U pull it wrecking yard. The cost was like $50.00 plus $5. 00 core.
When swapping to another rear finding a donor with disks would be a bonus.
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It's still the spider gears. Once the clutches in the sure grip unit allow one wheel to spin, the spider gears start getting abuse. If the unit is good and strong though, I "guess" the axles become the weak link. They're mighty small. Maybe it's a toss up between them and the pinion shaft. It's just that everything about the 7 1/4 is small. It's just not a performance rear axle. It never was. People have had good luck with them in that application, though. Just look at @Charrlie_S. So evidently, they're a little stronger than people give them credit for. I've driven mine around since I got it in January of 2019 and the guy I got it from drove it from when new, until 1980. It sat where I found it since that time.
 
Professor Fate said:
That's just silly what some people ask, when Dr. Diff offers this for the same or less money:
Mopar A-Body 8 3/4" Housing & Axle Package
(Yes, I know- currently out of stock, but you get the idea.)
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Thanks for the link. That’s good to know
Kern Dog said:
...OR be a weirdo and use two LONG sides to make an extra wide axle to copy Uncle Tony:

View attachment 1716063972
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ironically “Uncle Tony” is one of the few people to defend the 7.25 rear ends and in one of his videos on mopar rear ends actually say something positive about them.
 
1968 Dart 270 said:
ironically “Uncle Tony” is one of the few people to defend the 7.25 rear ends and in one of his videos on mopar rear ends actually say something positive about them.
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I'm the other one, except for the fact parts are made of unobtanium. They are lighter, and take less power away from the wheels (like a 904 compared to a 727). I wouldn't use a 7 1/4 behind a stick trans, and submitting it to shock loads, but they hold up pretty good with a built slant or mild 273/318 and a auto trans.
PS: the sure grip spiders are stronger because there are 4 spider gears, unlike the open rears which only have two spiders.
 
Charrlie_S said:
I'm the other one, except for the fact parts are made of unobtanium. They are lighter, and take less power away from the wheels (like a 904 compared to a 727). I wouldn't use a 7 1/4 behind a stick trans, and submitting it to shock loads, but they hold up pretty good with a built slant or mild 273/318 and a auto trans.
PS: the sure grip spiders are stronger because there are 4 spider gears, unlike the open rears which only have two spiders.
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My 68 Dart has a 1971 360 and 727 in it but has the stock rear end. How rare are sure grip 7 1/4” rears?
 
I do not think they were very common.
I would do something different. By the time you set up the new differential. Or replaced the whole rear end you might as well upgrade to something that matches the 360 727 combination.
 
1968 Dart 270 said:
My 68 Dart has a 1971 360 and 727 in it but has the stock rear end. How rare are sure grip 7 1/4” rears?
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Actually, they were not uncommon. I have run across quite a few. However, the ones that that are still out there have so much wear, the clutches are most likely worn out. I have not seen any clutches for sale for about, at least 5 years, and I bought those . I don't have those any longer, as I used them to rebuild a couple of rears which I sold.
PS: I do have a set of 5.13 gears for a 7 1/4 :)
 
DC Dart said:
I do not think they were very common.
I would do something different. By the time you set up the new differential. Or replaced the whole rear end you might as well upgrade to something that matches the 360 727 combination.
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Not as uncommon as you think.....at least in the sense of how many came in cars. They're uncommon now because people snatched them out and scrapped them.
 
No mention of the '66 B body rears? I have a housing with everything except the punkin. I Have a 3:91 peg leg in the shop.

My present 7 1/4 is a peg leg 3:23 with 67K on it. It hasn't moved since 2009.

Neighbor has a 8 1/4 in a van with 3:55's and a sure grip. He wants too much for the van. I guess I can gut the gears and put them in a A body 8 1/4.
 
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glockr said:
Thought I had a line on an A body 8.25 rear for my wagon but that fell through so I'm wondering if the 7.25 open diff would be strong enough for my use. Engine is a a mild Slant 6, but I'm going to upgrade sometime in the future to 10:1, hotter cam, and dual Webers on an Offy intake. Also I do kind of tend to drive agressively (or as my wife says, "stupidly") sometimes. I'm ordering a front disc brake kit next week and need to decide on 4 or 4.5" lug pattern. If the 7.25 is strong enough I'll go with the 4" to match the rear, but if I'm going to need the 8.25 (or 8.75) might as well get 4.5 since that's what the eventual rear will have. Thanks.
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The 7 1/4 is strong enough for your /6 with the upgrades you listed, even if you have a stick and pop the clutch off the line. What you don't want to do is start a burnout on a surface that has worse traction and then move forward onto a surface that has better traction while the wheels are still spinning, like from concrete to asphalt, because the sudden tire bite might cause the spider gear teeth to break.
 
str12-340 said:
I absolutely destroyed the original 7 1/4 in my 1970 Dart Custom in 1976, with a stock 318 2bbl. Broke the spiders and pieces went everywhere!!! Swapped in an 8 3/4 that is still in the car and doing fine, thank you very much. The car has had a 340 in it since 1978 and the 8 3/4 has over 150,000 miles on it...
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I also destroyed my 7 1/4 behind a 340 with a T5 five speed. Revved it, dumped it and every gear came off the ring. Has 8 3/4 now and T56 magnum. No more issues.
 
Rug_Trucker said:
No mention of the '66 B body rears? I have a housing with everything except the punkin. I Have a 3:91 peg leg in the shop.

My present 7 1/4 is a peg leg 3:23 with 67K on it. It hasn't moved since 2009.

Neighbor has a 8 1/4 in a van with 3:55's and a sure grip. He wants too much for the van. I guess I can gut the gears and put them in a A body 8 1/4.
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Hey Brother from another Mother! Good to see you're still kickin! :)

If you're still running up and down the road, holler at me sometime on the way thru.
You will have to swap the whole 3.55 carrier in an 8 1/4 if you have 2.76 gears or less in it now.

:thumbsup:
 
Has anyone tried cryo treating the gears? I've seen some marine transmissions get the treatment, but although I know it's done on vehicle transmissions and rear ends, I don't know anyone who has done that.
 
JDMopar said:
Hey Brother from another Mother! Good to see you're still kickin! :)

If you're still running up and down the road, holler at me sometime on the way thru.
You will have to swap the whole 3.55 carrier in an 8 1/4 if you have 2.76 gears or less in it now.

:thumbsup:
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What's up! I have a nephew over in Denton NC now. Last time I went was the Northern route up 81 down through Fancy Gap. He was in Kernersville. I hate getting stuck in the gorge and hate 40 through NC.

Funny thing my neighbor called to tell me was thinking of trading that van for a John Deere mower. He wants $500 for the van. 225 with a head that I did a "RV" porting on. Rebuilt a 2bbl and did an intake swap. It has the 833OD in it too. I could part a bunch of that out. Of course parts demand will dry up if I buy it.
 
Rug_Trucker said:
Sounds inexpensive! :rolleyes:
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Actually, it seems pretty inexpensive. I see a handful of companies having options to cryo treat gears they sell for $70 to $100. An article I found from MotoIQ in 2018 says "Cryogenic treatment generally can improve the fatigue strength of steel gears by over 100%. Cryo treatment is relatively inexpensive, usually the gears and shafts of a transmission cost around $100 to process." I imagine rear end gears would be in the same ballpark.
 
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jcwren said:
Actually, I don't think it is. I see a handful of companies having options to cryo treat gears they sell for $70 to $100. An article I found from MotoIQ in 2018 says "Cryogenic treatment generally can improve the fatigue strength of steel gears by over 100%. Cryo treatment is relatively inexpensive, usually the gears and shafts of a transmission cost around $100 to process." I imagine rear end gears would be in the same ballpark.
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It was sarcasm.
 
cryo treating in and of itself isn't that expensive, about $150 for a R&P, but when you start to figure in all the other ancillary items the return becomes less ideal. shipping a R&P back and forth, buying a SG unit if you can find one, buying another R&P in your desired ratio if needed. then all of the consumables to rebuild everything...

all of a sudden a whole 'nother rear end starts to look a little more enticing...
 
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