Making The Weak 7-1/4 Rear Axle Live... Kinda

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Ant

Ant

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Howdy!
I have a challenge for my self and would like some constructive input.
I'm building a 318 for my 73 Dart, it should make somewhere in the ballpark of 300HP. It will be bolted to the A904 auto transmission with a stock-ish stall converter. The car's weight is somewhere around 3450 pounds with driver.

So, I don't have a stronger rear yet, so the challenge becomes making the 7.25 rear hold up in the meantime. So far I know the spider gears in the diff are pretty weak, so let's start there. I have some ideas, like:
Welding the spiders together.(May not be good for street use)

Somewhow converting the open diff to a LSD.(Will be lots of research to be done for cheap)

Tuning the car to accelerate softer from a stop.

Not driving like an idiot.

Now before anyone replies with "Go buy a Dana 60 or Ferd 8.8" please understand that this is for fun and I'm not running a torque monster of an engine.
 
What gears do you have in the rear? Is this a small bolt pattern rear, or a 5x4.5?
I definitely wouldn't weld the spiders, especially on the street.
If you're not regularly hot doggin' it, it could last you a surprisingly long time- especially if it's a later large pattern unit.
 
If you can locate a 7 1/4 Sure-Grip that will be twice as strong as the stock One Wheel Wonder as they have 4 spider gears and 2 cross pins vs the 2 spider gears and single cross pin that usually wear out.

As long as the rear end will eventually be junk running 300 hp, a simple first step would be to weld in the cross pin on both ends so that it cannot work free in the case. Leaving the 2 spiders free floating running on center to do their task.

First point of failure is the cross pin working loose in the case. With that the 2 spiders go out of alignment and Boom, out of business.

☆☆☆☆☆
 
I've researched it in depth. There's nothing in the aftermarket that's close enough to modify, so there's that. Have you actually seen how small those axles are? I'm not trying to talk you out of it, you go ahead. I wish you luck.
 
My first car, a 1972 Duster, had a stock 318 with a 7 1/4. I worked after school pumping gas and helping the owner in a garage.

One day I power brake the car in the gas station parking lot and bang.....
Owner says "I'll order up some spider gears and you can help me replace them tomorrow". Mind you, this was around 1979.

We replaced them and I didn't do any more power braking. Lasted a few years until I sold the car.
 
If you can keep one wheel from spinning and then grabbing traction you might get it to last a long time. I've tore the spiders out of a bunch of 7-1/4 rears but it was always spinning a wheel, whether on ice and snow or an in an activity that could get me an exhibition of acceleration ticket.
 
I suggest skinny rear tires, maybe old bias ply's. Something that won't even attempt to hook up. 300 hp shouldn't destroy that 7.25 right away. Don't waste any time or money on that 7.25. There's a reason everyone puts something bigger in with a performance motor. You can stick your foot in it, just keep the burnouts to a minimum.
 
a 7.25 will take a surprising amount of abuse before giving up the ghost. if you abstain from john force peg leg smoke shows around town, fend off the urge to power brake jack rabbit starts while playing stop light eliminator, and avoid neutral drops you should be fine for quite awhile.

keeping skinny tires and freeway flyer gears helps.

the comical part is that you'll probably wind up busting it doing something mundane like turning into a drive way at low speed and having one tire slip in some gutter water and the other grab traction and go pop.
 
I knew an old man that put a 340 in his six cylinder Dart Swinger he left the 7.25 rear but he never accelerated hard with the car ..he drove it for 10 years without any issues...he babies all his cars
 
You can try this, back in the day my brother broke his 7 1/4 so he welded the spider and axle gears to the case and got an extra left side axle, ground the splines down enough to rotate in the axle gear freely and when he wanted a locker, he would change the axle with the splines, after I broke mine I went to pick a part and found an 8 3/4 drum to drum for 55.00 1979 price.
 
Weld a-lok
Lincoln Locker. There's others, I've forgotten. I spent a winter driving my 57 chev, winter of 67, with weld-a-lock. It teaches you a thing or two about driving dirt/ slick/ snow/ ice.

Makes interesting sounds on dry pavement as you pull out of a drive-in WITH THE COPS PARKED RIGHT THERE
 
I have a 7.25 in my 360/727 dart right now. I have driven it for a few years "babying" it and now that I have a 8.75 almost ready to drop in (no rush to swap it in at the moment) I have beaten the living hell out of it. I havent done any power braking but I tend to punch it at stop signs or while coming out of turns by my work when it's all empty on the weekends to whip out the rear. I regularly smoke the peg leg and she refuses to die. I was so scared to drive it at first and now I can't kill it...
My previous tires were really skinny (about 155mm wide) and my current tires (about 225mm wide) are really old and don't have much traction so I've never really made it "hook" either which I know helps the rear end live
 
Where in Ny are you? I have a big bolt pattern 8-1/4 I can sell you with a wratchet locker in it with 40,000 on it from a 74 Dodge Dart 318 car. All new seal and bearings from Yukon. Brand new cover and I dont have a picture but can get one on Friday . Let me know if you are game. $350 cash.
Syleng1
 
Professor Fate said:
What gears do you have in the rear? Is this a small bolt pattern rear, or a 5x4.5?
I definitely wouldn't weld the spiders, especially on the street.
If you're not regularly hot doggin' it, it could last you a surprisingly long time- especially if it's a later large pattern unit.
Click to expand...
5x4.5 with 2.76:1 ratio.
 
Syleng1 said:
Where in Ny are you? I have a big bolt pattern 8-1/4 I can sell you with a wratchet locker in it with 40,000 on it from a 74 Dodge Dart 318 car. All new seal and bearings from Yukon. Brand new cover and I dont have a picture but can get one on Friday . Let me know if you are game. $350 cash.
Syleng1
Click to expand...
Sounds good, but I need to find a complete sure grip carrier for it.
Also, that is a bit of a trip, like a few hours.
 
George Jets said:
If you can locate a 7 1/4 Sure-Grip that will be twice as strong as the stock One Wheel Wonder as they have 4 spider gears and 2 cross pins vs the 2 spider gears and single cross pin that usually wear out.

As long as the rear end will eventually be junk running 300 hp, a simple first step would be to weld in the cross pin on both ends so that it cannot work free in the case. Leaving the 2 spiders free floating running on center to do their task.

First point of failure is the cross pin working loose in the case. With that the 2 spiders go out of alignment and Boom, out of business.

☆☆☆☆☆
Click to expand...
Easier said than done. I have an idea for something, but I will need measurements.
 
7 1/4 rears are stronger than people give them credit for. They just don't like shock loading. Just drive it like a same person, untill you get something stronger. I ran 7 1/4 rears for many years (but they were sure grips) without issues. Even a low 13.second drag car (3200 lbs) with slicks and nitrous. Very hard to get parts for them, now.
 
I will need to take some measurements or if anyone has the measurements for the space between the spiders an stuff that would help. I found something that wedges itself as the gears catch and rotate it on the pin it.
If I can make fit, I might try it.

images.jpeg
 
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Ant said:
I'm building a 318 for my 73 Dart, it should make somewhere in the ballpark of 300HP. It will be bolted to the A904 auto transmission with a stock-ish stall converter. The car's weight is somewhere around 3450 pounds with driver.
Click to expand...
at 300hp and the cylinder pressure at same as before the build, your 318 at stockish stall will have about the same power as the stocker did, up to about 3000rpm; so I wouldn't worry too much about it. But I would highly recommend a Suregrip to split the torque between both spiders.

Typically tho, guys don't bring the cylinder pressure up on their 318 "performance" builds and if the pressure is less than stock, at stock-ish stall, then then the power down there will be LESS than stock, so then your conversion will be easier yet, on the rear gear.
Next, install enough rear tire so it can't spin at all at any rpm in first gear, and then the 7.25 will last forever, so long as you don't do silly stuff like neutral-drops and chit-hooks.
If your cone-type SG loses it's sure grippiness, it can be reconditioned. If you wait a lil too long, well, you wanted an 8.75 anyway .........
Two things kill the 7.25;
1) shock loads shatter the pinions, and
2) peglegging it welds the crosspin to the spiders. Make sure you install good oil and plenty of it for cooling.
 
AJ/FormS said:
at 300hp and the cylinder pressure at same as before the build, your 318 at stockish stall will have about the same power as the stocker did, up to about 3000rpm; so I wouldn't worry too much about it. But I would highly recommend a Suregrip to split the torque between both spiders.

Typically tho, guys don't bring the cylinder pressure up on their 318 "performance" builds and if the pressure is less than stock, at stock-ish stall, then then the power down there will be LESS than stock, so then your conversion will be easier yet, on the rear gear.
Next, install enough rear tire so it can't spin at all at any rpm in first gear, and then the 7.25 will last forever, so long as you don't do silly stuff like neutral-drops and chit-hooks.
If your cone-type SG loses it's sure grippiness, it can be reconditioned. If you wait a lil too long, well, you wanted an 8.75 anyway .........
Two things kill the 7.25;
1) shock loads shatter the pinions, and
2) peglegging it welds the crosspin to the spiders. Make sure you install good oil and plenty of it for cooling.
Click to expand...
Oh yes, peglegging can break the gears from shock if the tire grips after spinning. Getting a loaded Sure Grip carrier would be fine if it were not already kinda rare and service parts for it were easy to find. That is why I'm looking to do some experiments to find something to help with the breaking of the spider gear issue. Not everyone wants to spend big money for a 8-3/4 rear or a dana 60 just so they can have a peppy car. So finding something that at least kinda works would help me and others who try to stick with the weak 7-1/4.
I think the tune and setup may make a good difference as well.
Although, an 8-3/4 rear axle would be kinda cool.
 
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Some 8 inch Ford rears are close to a bolt in on an "A" mopar. If you can find the correct width they are not expensive. The Ford guys take them out for a 9 inch. I am building one from a 1976 Comet V-8. The spring perches are 43 inches, the overall width drum flange to drum flange is 56.5 inches. The mopar "A" body rear is 43 inches for the spring perches and 55.6 inches for the overall width. You get 10 inch brakes, and 5 on 4 1/2 bolt pattern. Even the mopar e-brake cables will fit.
 
Charrlie_S said:
Some 8 inch Ford rears are close to a bolt in on an "A" mopar. If you can find the correct width they are not expensive. The Ford guys take them out for a 9 inch. I am building one from a 1976 Comet V-8. The spring perches are 43 inches, the overall width drum flange to drum flange is 56.5 inches. The mopar "A" body rear is 43 inches for the spring perches and 55.6 inches for the overall width. You get 10 inch brakes, and 5 on 4 1/2 bolt pattern. Even the mopar e-brake cables will fit.
Click to expand...
That's great info, but I would like to keep parts like that from the same family for this build.
 
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