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Old 04-15-2018, 06:59 PM   #1
62 Barnfind
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GM 7.5 rear axle

I picked up a 70's era gM 7.5 axle today. I know it's probably not the ideal axle but the price was good and the length, (60in) and the bolt pattern (5x4.75) was right. Has anybody used this axle in a swap before in a 55-59? Just wanted to know if it will work or do I need something else. I will be running about 300 hp and from what I read the axle will be fine. I want to be able to drive long distances with it. Thanks!

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Last edited by 62 Barnfind; 04-15-2018 at 07:21 PM.
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Old 04-15-2018, 07:35 PM   #2
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Re: GM 7.5 rear axle

Im running a 7.5'' from an 87 Camaro in my 56. Ive logged almost 20K miles on it. It is what it is. A weak axle that wont tolerate large amounts of HP when mixed with good traction. I also have a 4cyl S10 T5 trans, so that keeps me from hammering on it.
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Old 04-15-2018, 07:39 PM   #3
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Re: GM 7.5 rear axle

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Originally Posted by cal1320 View Post
Im running a 7.5'' from an 87 Camaro in my 56. Ive logged almost 20K miles on it. It is what it is. A weak axle that wont tolerate large amounts of HP when mixed with good traction. I also have a 4cyl S10 T5 trans, so that keeps me from hammering on it.
From what I read 330 HP is about the max. i'll be running a 5.3 but no upgrades. I just want to go down the road without having a screaming engine.
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Old 04-15-2018, 08:33 PM   #4
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Re: GM 7.5 rear axle

70s era axles had numerically low gears because overdrive wasnt currently offered. Thats what you want if you dont have overdrive. Check the ratio. It could be anything from a 3.08 down to a 2.56 . Mine was from the od era and is a 3.42
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Old 04-15-2018, 08:44 PM   #5
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Re: GM 7.5 rear axle

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Originally Posted by cal1320 View Post
70s era axles had numerically low gears because overdrive wasnt currently offered. Thats what you want if you dont have overdrive. Check the ratio. It could be anything from a 3.08 down to a 2.56 . Mine was from the od era and is a 3.42
I don't know what year its from or the gear yet . I will have OD on this truck.
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Old 04-15-2018, 10:19 PM   #6
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Re: GM 7.5 rear axle

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From what I read 330 HP is about the max. i'll be running a 5.3 but no upgrades. I just want to go down the road without having a screaming engine.

you could probably have a lot more hp on a 7.5 as long as you dont have a lot more traction. every 7.5 I have seen with problems was from hooking up, not spinning, and with 330hp without any weight over the rear you will be spinning most of the time. I think you will be fine.
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Old 04-15-2018, 11:24 PM   #7
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Re: GM 7.5 rear axle

They used it in Camaro's with V8's for years. Even Z28's had it. If you take off hard with sticky tires it might brake. Should be fine for normal use.
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Old 04-16-2018, 08:25 AM   #8
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Re: GM 7.5 rear axle

It's really about the torque more than the hp. A 7.5 with good traction against 400 ft/lbs loses, regardless of the rpm the engine's spinning.

Different gear ratios also make a difference. The ring and pinion ratio multiplies the torque coming into the rear end. 300 ft/lbs through 3.73 gears creates 1119 ft/lbs at the axles. But 400 ft/lbs through a 2.29 gear only creates 916 ft/lbs at the axles.

This is part of the reason why the 7.5 in my '79 Monte survived for 8 years behind a Buick 455. The 455 was only producing about 370 ft/lbs and the rear had 2.29 gears. When I replaced the axle it was due to wear at the axle bearing surface.

Last edited by 1project2many; 04-16-2018 at 08:43 AM.
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Old 04-16-2018, 01:49 PM   #9
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Re: GM 7.5 rear axle

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Originally Posted by PDW HOTRODS View Post
They used it in Camaro's with V8's for years. Even Z28's had it. If you take off hard with sticky tires it might brake. Should be fine for normal use.
agreed. 1000s of bbc camaro/firebird and trans am of the day had them
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Old 04-16-2018, 08:14 PM   #10
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Re: GM 7.5 rear axle

Thanks for the reply! I figured that it would be ok for what I'm doing. I can still be on the lookout if I find something better. I start tearing down this Saturday to get real for the suspension build. Pics will be coming!
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