7.25 rears: Any mechanical engineers got suggestions for 7.25 rear end improvements?

[roadracerjeff]

Everyone says stay clear of 7.25 rear ends. Nobody makes posi's for the 7.25 -not that I know of anyway. However I've heard that the factory put 3.23, 3.55, and 3.91 posi's in super /6 Valiants and such. So, why can't I get one now? What makes the 7.25 "blow-up" -what's the weakest link(s)?? Does anyone know any tricks on how to beef up a 7.25 and where I can get a new 3.23 posi for one?? Before anyone chimes in telling me to replace my 7.25, that's not happening as the car is all original and staying that way. Surely, there must be some things that can be done to improve a 7.25...

[procommuter]

I had a 7.25 in my '69 Dart for over 15,000 miles before it scattered. The differential split right in half, which ruined the ring and pinion gears. The ring is now a clock in my office. I abused it alot, but I knew that someday I would have to replace it. I have seen repair procedures for the 7.25 suregrip in the FSM. By the looks I would say that would have to be much stronger than the open differential. But I have no idea where you would find one. And, yes I am a mechanical engineer.

[mdeane]

Having worked in a Dodge dealership parts department since the mid 1970's, I can tell you that the majority of problems we had were with the side & pinion gears inside the carrier on open-end units, seems like we would sell 2 or 3 rebuild kits a month back then. I can't remember very many problems with the sure-grip setups.

[dwc43]

The major problems are the size of hte ring and pinon gears. They are real easy to break. The bearings are too small and over heat and wear out quickly. The carrier is too weak and the axles do not have a large enough diameter and are very week, in the spline area mostly. They are very weak over all and if you do more than just drive it normal then it will fail.

[ehostler]

I once bought a modified '70 Challenger. It had a 383 4bbl, 4 speed, and an open 7¼ rear.

I took it out for some mild test and tune (nothing harder than 1 grand drops), one day. After we got it well tuned, we were driving back to base. About ½ mile into the drive, there was a loud thud and a noticible bumb. Looked in the rear view mirror and saw nothing in the road. About 500 feet later, the car skidded to a stop and the motor bogged down. Put it in reverse and backed up about 10 feet and then the same thing. Going back and forthe, I got to the point where the car would not go either direction.

I had her towed back to base. Once on base, I need to figure out how I was going to get it into the auto hobby shop, the next day, since I couldn't push it with a locked up rear. I went ahead and put it in 1st and then dropped the clutch at about 4 grand. One loud bang and then the sound of ball bearings in a blender...

Once It got the rear out, I opened it up and found that over half of the teeth in the ring gear were gone (whould have been caused before the 4 grand drop, as the car didn't move with the drop). All of the spiders were broken into pieces and the pinion gear was completely stripped of teeth.

I was lucky to have found a B body Dana 60 nearby, for dirt chea

[fastmopars .inc]

it scares me to hear someone would build a car with those pieces.
im quite sure that just regular driving with a big block 4 speed would take that diff out in a matter of days

[rustycowl]

Gotta love the way you remedied that locked up rear end! That's hillarious! I've broken them behind slant sixes enough times, I just yanked them out and replaced them every chance I had. I usually had trouble with the spiders, cross shaft, and carrier.

[ehostler]

It wasn't so much that he built the car that way by choice. It was originally a 318 automatic. He pulled the engine and tranny out of something else. He rebuilt the engine and the converted the car for a 4-speed. He then bolted it all together with hopes of finding a better rear at a later date. He either lost interest or didn't have the funds to finish the car, so he sold it.

I'll tell ya, it sure was fun freeing up that rear... It was the only time that a destroyed a MOPAR part by choice...

[dave571]

If the car is all original and staying that way, then what original engine does this have, that is so stout that it will rip up the diff it came with?

If you've beefed up the engine, why not keep the 7 1/4 in the garage, and throw in something better to abuse?

If you want to keep it original, does it malke sense to keep the original part, so you can destroy it, and replace it with a non original, out of necessity?

Not trying to start one of these but it just seems a bit confusing.

The ratio question would seem to be answered by this, Not all slant 6 cars came with 7 1/4 diffs. The better ratios probably came with a better rear end.

[dwc43]

I just broke one a month ago myself. Stock 318 2 bbl Volare'. Pulled out onto a rain soaked road and hit an oily spot. It spun just a little bit and when it hit a dry spot and caught it went BOOM!! Busted all of the spider gears and the ring carrier. Never got over 20mph at the time. These rears are junk for sure.

[Luther]

Smaller ring gear = less pitch on gear teeth = lower strength gear. An inevitable consequence of ring - pinion gear geometry. Also, since the gears are not going to be as strong as that of a larger rear end; a lighter housing and bearing design can be used to save $ s. No, have never seen the inside of a 7 1/4; but I am a ME and ChE.

[bg71361]

Replace it!

[barracudadave67]

69gtcuda318

I have a couple of those sitting around here if you want another one, you can have all the mounting hardware with it.Just come and get it.It came out of a 67 Cuda. I need the room in my Garage. They both still have the factory inspectors marks on them.

I am in southern Wis. if yuo are interested

Dave C.

[barracudadave67]

69gtcuda318

I have a couple of those sitting around here if you want another one, you can have all the mounting hardware with it.Just come and get it.It came out of a 67 Cuda. I need the room in my Garage. They both still have the factory inspectors marks on them.

I am in southern Wis. if yuo are interested

Dave C.

[vanderkwv]

I got lucky and bought a 72 340 Duster (less the engine) for $800. and swiped the 8 3/4 and put it it my "other" 72 Duster. I then put the 7 1/4 back into the 340, I bet it was the only 340 Duster with an 8 3/4

[vanderkwv]

Opps, I meant to say "I bet it was the only 340 Duster with a 7 1/4 rear"

[George G. Leverette]

I see these really weak 7 1\4 rear ends in cars in the junk yard where every thing except the rear end gave up. There is a 72 A-body with 318 in the local yard with a good 7 1\4. I also have a 7 1\4 from a 68 barracuda with 318 available for the asking but I live in California. Maybe Synthetic rear end lube would help, I use Mobil 1 dont really know the benefits it provides but it dosen"t stink and works better at low and higher temperatures according to mobil 1. The lower the gear the thicker the teeth on the ring gear. There were sure grips for 7 1\4 rear ends, contact one of the vendors that deal in rear ends. Every one dosen't drive 1\4 mile at a time but that thinking appears to prevail regardless to what question is asked.

[dwc43]

The synthetic gear oil only has one benifit. IT will take higher temps better BEFORE it starts to break down. It wont help the little 7 1/4 axle from comming apart. I've torn up too many of these things just ridding around town. They just dont hold up. They are just not designed right.

[wiesshund]

I had a 72 dart 225 slant 6
the 6 was modded, had 3 webbers and a header, bored etc
the lil 7 1/4 rear never failed

I think youd be ok with anything from a mild 340 and down
no they arent drag rear ends and there isnt enough metal to support extreme stress on them, but they ran for 100's of 1000's of miles of normal road driving.

Its when we play don garlitts that they blow up.

SO question is, how nice will you be to it?
All that said, id rather have an 8 3/4, not to mention a more normal bolt pattern

[Drews'70]

In the 70 Dart I had with the 71/4 rear, auto trans, 360 I never had a problem at all. I ran the snot out of it. It wasn't a sure grip but every now and the she'd lay down 2 strips. I was waiting for the day she'd blow cuz all I heard was, get an 8 3/4 or you'll be sorry. Could be a different story with a 4 gear.

[dwc43]

The major problems are the size of the ring and pinon gears. They are real easy to break. The bearings are too small and over heat and wear out quickly. The carrier is too weak and the axles do not have a large enough diameter and are very week, in the spline area mostly. They are very weak over all and if you do more than just drive it normal then it will fail. Ive blown more than one 7 1/4 behind a stock 318. Last one spun the right tire in the rain when it caught traction on a dry spot of pavement, it borke the spider gears, shaft, and drive gears, which chipped the pinion and ring gear. This was under normal driving conditions as I was just leavign the driveway to go to work. There not worth wasting you time on them. Replace them with a superior 8 1/4 or 3/4 and it will last up to 500 hp or so.

[George G. Leverette]

The ring and pinion are the weakest link on all rear ends, I have seen chunks out of many 8 3\4 rear ends with 323, 355 and 391 gears. Dropping rear ends at the yard will provide good insight into what breaks and on what type cars, trucks and vans. The 7 1\4 is fine if used for its intended purpose. The only reason I have 8 3\4 rears in my cars is because they were readily available about four years ago, passed up quite a few because of the hassle of removing them in the yards.

[roadracerjeff]

Originally posted by barracudadave67
69gtcuda318

I have a couple of those sitting around here if you want another one, you can have all the mounting hardware with it.Just come and get it.It came out of a 67 Cuda. I need the room in my Garage. They both still have the factory inspectors marks on them.

I am in southern Wis. if yuo are interested

Dave C.

Thanks to all who've replied here. FYI, my pc was down and that's why I haven't replied back.

Hey barracudadave67, if you're still watching this one, I may indeed be interested as you are not very far from me. If I understand you correctly, you have a couple of sure-grip 7 1/4 rear ends. If so, would you happen to know ratio(s) and anything about condition? Thanks,

[roadracerjeff]

Originally posted by dave571
If the car is all original and staying that way, then what original engine does this have, that is so stout that it will rip up the diff it came with?

If you've beefed up the engine, why not keep the 7 1/4 in the garage, and throw in something better to abuse?

If you want to keep it original, does it malke sense to keep the original part, so you can destroy it, and replace it with a non original, out of necessity?

Not trying to start one of these but it just seems a bit confusing.

The ratio question would seem to be answered by this, Not all slant 6 cars came with 7 1/4 diffs. The better ratios probably came with a better rear end.

Good points for a stock engine, but mine won't be stock -it will only look stock. 3 things are driving me to do this with a lowly 318 2-barrel. It started when I read a great article about someone getting 350 HP and 316 FT LB out of a highly modifed Ford 289/302 engine with 9.25 to 1 and a 500 CFM 2-barrel. Next, I became aware of the F.A.S.T drag class. So, I then thought it would be cool to see just how much power I can make with a totally stock looking 318 2-barrel, single exhaust set-up (what came on the car). The third reason is more of a hunch than anything, and that's that we're headed for a real gas crisis in the not too distant future -I could be wrong. So, the 2-barrel with highway gears is really the way to go for what I want to do -more about that in a moment. Going back to what kind of power I'll have... I'd like to see 275 HP at the wheels with low 300's FT LB -no way I'll hit the numbers that they got with that Ford as they used headers, a trick, albeit small carb, and many other things that would deviate from the stock look concept.

Now, as for what I plan to do with the car. Drive it to shows all over the country, and entering in stock class. No regular drag racing is planned, but it would be nice to be able to get after it here and there with an original looking rear end that won't break. And, it sounds like the 7 1/4 sure-grip would do the trick given one of the replies from someone who worked a parts counter that said they sold rebuild kits for 7 1/4 opens all the time, but rarely anything for 7 1/4 s-g. I understand that could simply be related to the volume of opens sold vs. s-g, but I'll take my chances. Enough for now. I'm sure I've evoked a number of different questions, and many will think I'm just wacked -all of which is fine. Not the goal though, my goal was to get feedback about the 7 1/4, and I got just that -even found a potential source for a sure-grip or two. Take care.

[Slant Cecil]

The weak link is the spiders. They make up about 80% of 7.25 failures. The hard to find Sure Grip unit will spread the spider gear load doubling the strength. If Sure Grips and a good selection of gears were readily available, I'd run a 7.25 behind my slant6, they are 110lbs lighter than an 8.75.

Cecil