MONTE CARLO How does a 7.5 rear end die?

Sep 22, 2023
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I've heard that with the 10 bolts rear ends the axles will walk out and destroy your car with them. I was wondering if this is true for the 7.5 as well.

I'm current building a 84 monte carlo ss that's gonna have around 450ish crank hp with a 350 sbc. And I want to know if just sending the rear end without replacement is just a totally bad idea. If it's gonna explode and take me and the car with it I'm definitely gonna look into a new rear end.
 
Two issues develop (other than c-clip failures which are not common) : pinion and pinion bearings can’t take hard hits, 2nd and most common, the axles are puny and break.

Most axles that come out are due to axle breakage. Wheel hop and sticky tires that don’t spin on a prepped surface are the enemy. A good set of aftermarket axles, 28 spline, with a 28 spline center section will live at 400-450 wheel hp if the rear end is setup properly. But it will be the weak link in the chassis once the transmission is upgraded.
 
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BEFORE you spend your first dime on anything, sit yourself down and figure out what your end product will be. Street, street/track or strictly track? How will it be driven? If a track or sometimes racer, how fast will it be? Yes, a 10 bolt can be built to take moderate power but anything over 250/300 will need some re-enforcement or a different / better setup. Curie, Strange and many others make a variety of differential units that will fit. Or, you can build your own. Blowing up a rear generally is a self contained failure. C clip axles are banned by the NHRA. There are eliminator kits to make them safer but I've never seen a kit that didn't leak. Personally, I use Ford 9" diffs.. They are very strong and reliable, especially when fitted with a detroit locker or a spool (track only). Working from a plan, will save you lots of $ in the long run. Use parts that are stronger than you think you need and you won't be repairing or replacing stuff every week. Building is like writing music. ... It all has to compliment each part and when everything works together, you have something special. Also note: Race gears are for racing only. They are actually softer than street gears and will quickly wear out in a street car. If you look on the used parts sources online you can get great deals on stuff if you're patient.
 
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I've been surprised by my 7.5" rear in my road race Camaro. I would have thought it'd be more problematic but at 260whp/310wtq, 3250-3300# post race (with me), and DOT legal R compounds, they're bulletproof. I'm certain if I had proper traction in a drag racing environment my tale would not be the same, but if you're in a street car that doesn't hook like crazy you can get away with quite a bit more.
 
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I have put a 7.625" rear into the 10's with a 510rwhp setup, got two passes in with a 3200lb car, on the third try I blew the spider gears out the rear cover during the launch.

I have stripped the pinion gear on a 7.625" setup with drag radials on the street, with a 400rwhp setup.

Just depends on the day..
 
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I think the best advice is to 'fit the punishment to the crime'. By that I mean if you are going to put a high powered engine in front of a 7.5, with big sticky tires, and abuse the #$%^ out of it, then expect it to fail. So upgrade accordingly. A stock 7.5 is not a piece of junk, it is just a slightly weaker version of the tried-and-true 8.5, which was introduced because of the general downsizing that occurred in the late 1970's. I personally used a dead stock 7.5 with the gov-lock LSD in my 1981 Malibu wagon. I had a 1970 Pontiac 400 with a pretty strong cam and a 4 speed Saginaw manual trans. I was NOT easy on it! The reason it lasted was because my first gear was 3:50 and my rear was 3:23, so my overall ratio was 11.3 to 1. That takes the strain off the rear and trans. If you keep the stock automatic gears like 2:41, or 2:56, and then try to push them with a big motor, something has to give. I just experienced something similar with my outboard motor. The prop I was using was great for top speed. But when I changed the lower unit oil at the end of the season, it was usually black- meaning burnt. The last couple of years I have been using a prop with 4" less pitch. It had less top speed, but easily carried more weight, and the oil came out fairly clean. Same principal- take the load off.
 
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I'll throw out my experience here. I had an '84 Cutlass with a 408 BBC in it with a th400 and nasty shift kit. I blew two factory posi units all to heck doing burnouts and twisted 2 sets of stock axles. This was with 3.42 gears. I got mad and bought an auburn posi and at the time 30 spline axles that were for a different application but fit the 7.5. That settup handled some nasty abuse from me including zero spin launches at the track and 13.3 1/4 miles. Not a super fast car, but it did weigh about 3900 pounds with me in it. I drove that car for 15 years and no trouble from that rear end settup.
 
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What dif. carrier number would a 2.73 be. I have a 2.73 in the car now and a posi with 3.73 gear on the shelf. Is there a way to use the 3.73 posi carrier and the 2.73 gear. Did one 20 years ago and used a spacer but don't remember which way it went.
 
What dif. carrier number would a 2.73 be. I have a 2.73 in the car now and a posi with 3.73 gear on the shelf. Is there a way to use the 3.73 posi carrier and the 2.73 gear. Did one 20 years ago and used a spacer but don't remember which way it went.
Other way around. You can use series 3 gears with a series 4 carrier if you install a spacer. And some companies make 4 series gears to fit in 3 series carrier.
 
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Another problem with the 7.5" and it's C clips. Since the axle is retained by the C clip. Any breakage beyond the clip, like the axle itself will allow the axle, drum, wheel all to come out of the car together. This is basically why it's banned by NHRA.
If your axle is retained by the wedding ring and bearing at the outer flange. An axle break inside the housing will not allow the tire to leave the car.
 

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