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My uncle is currently going through his 1974 Nova and replacing/upgrading parts. Not the best pics but this is the car.

He wants to go from an open diff 10 bolt to 12 bolt "Posi" rear end. I told him from what I understand the Truetrac is better. The car is just a cruiser, no drag racing or anything. Currently has a 350 in it but may upgrade to a 383 stroker.

Anyway, any idea what the correct stud size is for his stock drums? He doesn't have a caliper and I am helping him remotely. I assume the first bolt pattern is correct for his car?

Also on gear ratio any recommendations? He doesn't drive it very far so doubt it will be going 70 on the highway for very long at all but he doesn't want too aggressive of a gear ratio. Maybe stock or a notch or two more aggressive. Any suggestions? He said all he cares that it's a 12 bolt and has "positraction" or equivalent. Any help is appreciated as obviously we are not rear gear dudes. Lol

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may be talk to ' Miles ' . . . . . . member name = JBimmolate = " The Resident Differential Guy "

or several other great SNS members should check in . (my rear-end is stock = I need too loose weight)

alot of guys use the 3.73 ratios . . . on the street . What type trans - does he have ?

jim
 
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I think you have the first 2 answered for a stock bolt pattern and stud size. If you go longer it may require open ended lug nuts but its not a huge issue. For gears, I think a 3.42-3.55 is an all around good driving gear.
 

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The 8.5" 10 bolt in his 74 is just about as strong as a 12 bolt. I would not waist money on replacing it with a 12 bolt for the way you say he will be driving it, even with the 383 upgrade. Just put a good posi and gears in it, and maybe some good traction bars. Use the money saved on other things
 

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The 8.5" 10 bolt in his 74 is just about as strong as a 12 bolt. I would not waist money on replacing it with a 12 bolt for the way you say he will be driving it, even with the 383 upgrade. Just put a good posi and gears in it, and maybe some good traction bars. Use the money saved on other things
We called a local axle place here in San Antonio. Said the 10 bolt isn't compatible with truetrac, but he said a positrac is. Said he can tear into it and give us a better cost estimate before proceeding but he said $1500 should cover it. You think that's an OK ballpark price?
 

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The 8.5" 10 bolt in his 74 is just about as strong as a 12 bolt. I would not waist money on replacing it with a 12 bolt for the way you say he will be driving it, even with the 383 upgrade. Just put a good posi and gears in it, and maybe some good traction bars. Use the money saved on other things
Also new Moser 12 bolt with truetrac is about 4K delivered. We agree, overkill for a cruiser. Money would be better spent on other upgrades.
 

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The 8.5" 10 bolt in his 74 is just about as strong as a 12 bolt. I would not waist money on replacing it with a 12 bolt for the way you say he will be driving it, even with the 383 upgrade. Just put a good posi and gears in it, and maybe some good traction bars. Use the money saved on other things
Yep, I agree...I run a low 11 second 1/4 mile '72 Nova with a '73 8.5" 10-bolt in it...shoe box full of timeslips with it and have not hurt it.
It is currently getting an upgrade from stock 28 spline axles to Dutchman 30 spline(like a 12-bolt) and a spool so I don't break the original stuff trying to go faster.

Denny's Driveshaft is one source for a helix gear/torsen style diff like the Trus-Trac, but you have to also upgrade to 30 spline axles if you want to use one....it is made for the current 8.5/8.6" rear GM uses, that is why it is only available in 30 spline. If your plans already include new axles for the 8.5" 10-bolt, might as well go 30 spline when you do and then you can use this.
It's called a Trac-Rite, from American Axle(AAM)

Stock wheel studs were 7/16"-20 x 1.25" long.
The longer 2-3/4 to 3-1/2" long stuff is typically used for racing where rules say the stud must protrude into the hex portion of the lug nut when using aftermarket wheels.
 

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Also, you can use stock '68-72 Nova 12-bolt replacement axles if you have the shop trim 1/16" of thickness off the c-clip button on the end of it, to make it match the thickness or the 10-bolt axle button.
 

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so is he running a 2.73 or worse gear at the moment?...a posi with a 3.55 would be a big improvement for a 160 hp motor if it makes that much ...if he goes 383 at 350hp ...got to change the converter also to let it spin up a little so it doesn't blubber
 

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Yep, I agree...I run a low 11 second 1/4 mile '72 Nova with a '73 8.5" 10-bolt in it...shoe box full of timeslips with it and have not hurt it.
It is currently getting an upgrade from stock 28 spline axles to Dutchman 30 spline(like a 12-bolt) and a spool so I don't break the original stuff trying to go faster.

Denny's Driveshaft is one source for a helix gear/torsen style diff like the Trus-Trac, but you have to also upgrade to 30 spline axles if you want to use one....it is made for the current 8.5/8.6" rear GM uses, that is why it is only available in 30 spline. If your plans already include new axles for the 8.5" 10-bolt, might as well go 30 spline when you do and then you can use this.
It's called a Trac-Rite, from American Axle(AAM)

Stock wheel studs were 7/16"-20 x 1.25" long.
The longer 2-3/4 to 3-1/2" long stuff is typically used for racing where rules say the stud must protrude into the hex portion of the lug nut when using aftermarket wheels.
Thank you for this! The diff guy is telling us if we have the 8.5" we should be able to with either AAM, Eaton, or Motive Helical. He said we are right on the cusp from transition from 8.2" to 8.5", so fingers crossed that we don't have the 8.2"!
 

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Discussion Starter #12
so is he running a 2.73 or worse gear at the moment?...a posi with a 3.55 would be a big improvement for a 160 hp motor if it makes that much ...if he goes 383 at 350hp ...got to change the converter also to let it spin up a little so it doesn't blubber
No idea, we may have to wait until the diff guy verifies. I'm not too sure on the history of the car as my uncle is not the first owner. I don't think even he knows what all is factory from the car. We assume it's whatever came on the car but not sure. My uncle doesn't know anything about gear ratios so he's never tried to ID which gears he has. He has a rebuilt 350 engine with some tweaks, like I know the cam is upgraded but even my uncle does not know which cam it is lol. Not sure how much HP he is making now, he is shooting for high 400's maybe low 500's. One turnkey 383 he was looking at claims 525HP (although not dynoed). He plans to get a new turbo 400 (something like that) trans when he replaces the engine. I think we can match the torque converter at that time to make sure we get the optimal setup.

 

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At a minimum use 1/2 inch studs. And once you get into the low 10's a 5/8 stud is a nice investment.
The car will never go that fast. It may have the potential to go that fast at some point but never will actually be used to it's full potential. My uncle = old man. He's not really THAT old but he drives like it! haha
 

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The car will never go that fast. It may have the potential to go that fast at some point but never will actually be used to it's full potential. My uncle = old man. He's not really THAT old but he drives like it! haha
I would steer clear of that ebay special you showed for a motor (intake wasn't bolted on) might wanna go look around a Chevrolet performance site. They have a lot of variety performance wise ...but nice street manners (such as vacuum brakes) if he has that...they will work
here's the link Crate Engines: Classic, Race, and Project Cars | Performance
 

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I would steer clear of that ebay special you showed for a motor (intake wasn't bolted on) might wanna go look around a Chevrolet performance site. They have a lot of variety performance wise ...but nice street manners (such as vacuum brakes) if he has that...they will work
here's the link Crate Engines: Classic, Race, and Project Cars | Performance
Ok, thanks. We will check it out. What does it mean that intake wasn't bolted on? I think the main reason my uncle liked it was for the looks. He liked the eye candy and custom intake. I don't know very much about engines, nor does my uncle.

We have been looking at the Blueprint engines on their site and from summit racing. How is something like this?

 

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Thank you for this! The diff guy is telling us if we have the 8.5" we should be able to with either AAM, Eaton, or Motive Helical. He said we are right on the cusp from transition from 8.2" to 8.5", so fingers crossed that we don't have the 8.2"!
If the rearend guy is telling you that you are ( right on the cusp from transition from 8.2" to 8.5" ) I would not use him, he doesn't know that the 8.5" came out in the 1971 Camaro, and in 1972 Novas, and other GM cars, it was GM's replacment for the 12 bolt, If it is the orginial rearend in his 1974 it will be an 8.5". I would go with the Eaton Posi. Look through this and you might be able to get the numbers off the axle tube and know what gears it came with, no guarantee that they haven't been changed though, also it shows you how to tell the difference between the 8.2" and 8.5"

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The 8.5 is more then enough axle for the type of usage it sounds like this car will see. An 8.2 is still plenty strong for your uncles needs. The 7.5 would also be fine but most us wouldn’t want to put much money into a 7.5 axle. The 8.5 is nearly the equivalent to the 12 bolt passenger car axle. They have the same size pinion gear shaft which make them pretty close strengthwise. Tha 8.5 axle housing and 28 spline axle shafts will be a little bit lighter by maybe 100lbs. The heavier duty passenger car axles like the 12 bolts for 67-72 were used mostly for high hp and toque engines of the day. Drag racing was the primary way most muscle cars were exercised and the launch is very stressful on the driveline and axles. The 12 bolt was GM’s big boy axle. The 8.5 and 8.2 10 bolts are lighter and buildable axles that are more then capable for what most of us do with our cars..

You can visually identify which 10 bolt axle is under your car.
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