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are GM 7.5 rears really bad?

16K views 22 replies 8 participants last post by  Augusto 
#1 ·
if they are not good, why they make lots of aftermarket gear sets for them?

I have an '81 Malibu that came with 2.73 or so and want to install a 4.10 gear set.

Will the rear end with 4.10 gears be more strong than it would with a lower ratio? it makes sense to me since there will be more mechanical advantage and will have less resistance to turn, therefore will last more.

am I correct?
 
#2 ·
Augusto said:
if they are not good, why they make lots of aftermarket gear sets for them?

Will the rear end with 4.10 gears be more strong than it would with a lower ratio? it makes sense to me since there will be more mechanical advantage and will have less resistance to turn, therefore will last more.

am I correct?
First; They maye them because ther a lot of Chevies out there and people buy them.

Second; the 4.10 is a lower ratio than a 2.73, just like low gear in a TH350, 2.53:1 is lower than high gear, 1:1. Will the 4.10s be stronger? I doubt it. It still has only a 7.5" ring gear. The 7.5 won't last if it's subject to any kind of thrashing. Even an 8.2 is considered inferior to a lot of other rears. I understand that the 8.5 is quite durable, but I can't confirm that because I run a Ford 9".
 
#3 · (Edited)
Augusto said:
...Will the rear end with 4.10 gears be more strong than it would with a lower ratio? it makes sense to me since there will be more mechanical advantage and will have less resistance to turn, therefore will last more.

am I correct?
In this case, I'm thinking not. Because the pinion gear is a smaller diameter, it seems to me that there's be less tooth surface area engaging the ring gear, so there will be a higher average loading on the pinion gear teeth.

But whether that's the case or not, it's really a matter of getting traction or not getting traction. If traction is sufficient, the 7.5" pinion/ring gear will fail sooner than a larger set of gears (all else being equal)- regardless of the ratio.

As far as there being good aftermarket support for the 7.5", it was installed into millions of vehicles, some quite sporty. Not everyone is going to want or need a stronger diff than the 7.5", so there's a good selection of gears available for that segment.

On the other hand, because there are so many vehicles that came w/the 7.5" rearend that have been modified, the aftermarket also has a good selection of replacement rearends: Ford 9", 12-bolt, and Dana types, along w/a lot of info on swapping in other rearends.

It's a damn shame GM used the 7.5" instead of the corporate 10-bolt 8.5" rearend in almost all of the G-cars and and 3rd. gen F-cars. If not for that, these cars would be even better for hotrodding. :mad:
 
#6 ·
well, I have a rear from a 1978 C10, it should be a 8.5, right?

I would have to add the tabs and stuff to adapt it to the G body's 4 link, not much of a big deal, if this is a much better option I think it's worth the hassle.

I also have another 8.5 from a 1970 camaro, but is missing the pinion yoke and I haven't found a replacement yet.

are both the camaro and c10 rears the same? wich one would you reccomend?

it cost the same the 4.10 gears for the 7.5 and the 8.5, so makes no sense spending in something that may break, the engine is a 144 B&M supercharged 355 SBC.
 
#7 ·
78 C10 should be an 8.5.

70 Camaro is probably a 8.2 Chev. If it is an 8.5, it will use the same yoke as the C10 axle, with the correct u-joint installed in the driveshaft

The 8.5 is alot stronger than the 7.5.

I usually see a few broken 7.5 per week. Most of them already have aftermarket axles, so I cant say much about the durability of GM axles.
 
#9 ·
Some of the last several years of the S10 used a 8.5 axle in a 4x4 version that will be about the right width for a G body.The other option is finding a Grand National Buick axle. Or go for a somewhat expensive. converted 9 inch Ford.
A properly set up 7.5 will take more abuse that you think....but it will break eventually from that abuse.
 
#11 ·
definitely both my C10 and Camaro rears are 8.5 units, according to the id pictures they are not 8.2's

the C10 has 5.5 bolt pattern and the Camaro's 4.75

the C10's is newer and looks in better shape, can I use the Camaro's axles in the C10's housing, so I won't have to drill new holes?

also a very important question about the width, I don't mind if the rear end is wider, I can use different wheels to compensate somewhat, but the real problem may be: IS IT VERY IMPORTANT FOR DRAG RACING TO HAVE A NARROW REAR END? or is it only a matter of fitting very wide slicks under the fenders?

I can't use a very narrow rear anyway, the frame rails will not let me, unless I start doing some serious work, but if it is a must then I will do it.

thanks for the help, the links have enough reading material to keep me bussy for hours. :thumbup:
 
#12 ·
A couple thoughts...

IIRC you are building a SBC turbo drag car. Depending on how hard this engine combo hits the tires and how good the traction is, there could be a need for more strength than a stock 8.5" diff can handle- but the basic 10-bolt 8.5" rear end (with mods as needed) has been used behind some damned gnarly engines (single-digit et turbo Buicks, for instance).

The rear end width isn't that important at least not until you start getting up there in power and traction. In cases like very high hp drag cars, often the rear ends are narrowed to fit enough tire under the vehicle w/o needing a lot of positive wheel offset. Having excessive offset can cause the wheel to flex, giving traction problems as well as the chance of breakage. A side benefit of narrowing the rearend is it will add some strength because the axles are shorter, but I wouldn't say narrowing a rearend that can fit enough tire for the job will be an especially useful modification- unless the wheel offset becomes too severe.

Guys will weld the housing to the tubes, swap out the housing ends for Ford-type bearings to get away from the C-clips, etc. All things that may or may not be needed, depending on things like the weight, manual or AT, size of the tires/traction, slicks or treaded, and of course the engine output.

If this is a track-only deal, I'd suggest you look into a full spool for it, along w/stronger axles (w/more splines and larger wheel studs) if it turns out the stock 8.5" diff/axles/rearend housing isn't enough.
 
#13 ·
yes, the engine is a supercharged 355 SBC w/manual transmission

I'm removing absolutelly all the stuff that is not necesary, this includes glass, doors (only the skin remais) interior, etc etc, I'm making it very very light.

this will help the rear end survive some abuse, less inertia means more longevity.

also I will be running street tires, P305/50R15 rears, no slicks.

I want to run a spool also

I was concerned about the rear end width because I tought wider tracks made the cars wander down the strip, not having to narrow it (for now) will let me concentrate on other important aspects of the build.
 
#14 ·
Augusto said:
...supercharged 355 SBC w/manual transmission... *SNIP* ...I was concerned about the rear end width because I tought wider tracks made the cars wander down the strip, not having to narrow it (for now) will let me concentrate on other important aspects of the build.
Ahh, that's right- roots blower, not a turbo. :cool:

The narrowed rear track WAS found to help early front engined rails track straighter. Those rails had a low c/g and roll center that didn't present an immediate danger for having a roll over problem, so the need for a wide rear track wasn't there.

IMHO the need for a narrow rear track isn't really a concern in your case being as how you are using a rear suspension and are not so severely over powered and under "traction-ed" as those rails were.

This is an article by Billy Shope of Ramchargers fame. He knows a LOT about the subject.
 
#15 ·
thanks a lot for the link, there's a ton of useful info in there, specially the calculators are incredible, you can clearly see the effect of making changes.

also I found out that my C10 is actually a 12 bolt, camaro's still 10 bolt.

I guess my best choice is the C10's, is a lot wider though, I will have to make it shorter somehow, also will have to build some sort of brackets or something to bolt the upper links, something will have to be welded to the tubes and run up over the pumpkin.

here are some pics of my rears:
 

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#16 ·
The truck rearend you have is not an 8.5", it is a 12 bolt 8.75", and a very good rearend for what you want to use it for. That rear should be a 5x5" pattern, not a 5x5.5" pattern. The rearend on the right appears to be the 1970 8.2", and not the '71 -'81 8.5" rear. The 8.5" rears have squared corners cast into the housing that hang below the rear and identify them as the 8.5" rears.
They look like this, my '71 Camaro 8.5":
 
#17 ·
indeed, it's a 5x5 pattern.

so it's a good one, great, I'm gonna use this one

do you know if I can weld the ears for the 4 link to this pumpkin? if the axle tubes can be welded to the pumpkin, I guess some 1018 steel brackets can be welded to it, so I can use it in my G body ?

what kind of weld? ER70ES MIG, 7018 stick?
 
#20 ·
cobalt327 said:
I'm no expert, but the only 10-bolt stock pinion w/30 splines that I'm aware of is the corporate 10-bolt 8.5".
If it's a 30 spline, then it should be a 8.5", as the only other 30 spline was the later Yukon 8.6". They did make upgraded carriers and axles for the 8.2" that made it a 30 spline, but they were aftermarket.
Make sure you've counted correctly, and it's not a 28 spline.
I am not a qualified welder, so I can't tell you what rod or wire to use to weld to the housing. I have seen people weld to cast steel housings, but not sure what they used.
 
#21 ·
I can take it apart and measure the ring diameter, what diameter is it? the outermost diameter of the ring or the inner diameter that mates to the carrier?

I'm pretty sure it is a 30 spline, checked several times, and had been looking for a yoke for it with no luck for months.

so it is possible to weld ears to the housing, good, I need to find out what rod and if 1018 steel can be welded to it.
 
#22 ·
Augusto said:
I can take it apart and measure the ring diameter, what diameter is it? the outermost diameter of the ring or the inner diameter that mates to the carrier?
Outermost diameter.

Also an 8.5" 10-bolt corporate ring gear is held on by 7/16-20 left hand threads, having a 3/4" hex head on the bolts. The 8.2" rear has 9/16" hex head bolts and is right hand threaded. The thread size of the 8.2 ring gear is 3/8-24.
 
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