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Discussion Starter #1
I have been sitting on this info for while and have only seen a hint of it in the truck sections.

We all know the GM 8.5 corp diff.vers the 8.875 and 8.2
Did you know that the 8.5 pinion is the same physical size as the 12 bolt pinion?

But what is not be known is that the lenth and diamiter of the axles for a givin car are the same within a 1/16" or less (in lenth) for a givin rear. In other words lets take a 1972 Nova, It could have came with a 12 or 10 bolt rear. 12 bolt axles have 30 splines, 10 bolt have 28, but yet both axles are the same lenth and diameter.

That means that if the 10 bolt had a 30 spline carrier the 12 bolt axles would bolt in. Now the trick. use a 1990 up gm pickup 10 bolt carrier it has 30 splines and bolts into the 72 nova housing with adapter bearings.

This is what I did, I ordered a 30 spline Truetrac for the 90 up Pickup and the adapter bearings from DTS. Then I ordered some Moser "C" clip replacement axles for a 1969 Camaro (easier) 12 bolt with moser "C" clip eliminators.

These parts all bolted in to my Stock 1972 Nova housing and I topped it off with a LPW cover just for that added Grunt. It has now been in the car and running for over 1000 miles and is SWEET. I fear no axle breakage or c clip loss.

Just for added grins I installes 1/2" 20 X3 screw in wheel studs topped off with the 79 Trans AM rear disk brake set up and it was all done in my little 2 car garage.

I have pics but are too large to post. So I will email them to any one who wants them.

This also applies to 68/72 chevelles (must to use the 72 8.5 diff for 71 and back), and 70/81 Camaro/Firebird . I don't remember the year split for the Camaro.



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veno said:
In other words lets take a 1972 Nova, It could have came with a 12 or 10 bolt rear.
1971 was the last year for the 12-bolt in a Nova. All 1972-75 Novas have the 8.5" 1976-79 Novas could have an 8.5" or a 7.5" 10-bolt.

That means that if the 10 bolt had a 30 spline carrier the 12 bolt axles would bolt in. Now the trick. use a 1990 up gm pickup 10 bolt carrier it has 30 splines and bolts into the 72 nova housing with adapter bearings.
Yes, aftermarket 30-spline spider gears are available for the 8.5" 10-bolt but I would rather install aftermarket 30-spline axles instead of stock 12-bolt axles.
 

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Discussion Starter #3 (Edited)
Moser

You should have read a little further down when I SAID " got Moser "C" clip Axles" I stand corrected on the year. I was just trying to pass on some info I found out, and trying to help others not wanting to spend 2000+ on a 12 bolt. I built my 8.5 including the disk brake setup for a shade over 1200.

These are the Replacement stock style axles but are not a Taper axle, and are forged of the same 1541H alloy as the custom axles, just the replacement axles have a 1.443 stock style bearing instead of the race style which is the 1.55 bearing.

NovaResource said:
1971 was the last year for the 12-bolt in a Nova. All 1972-75 Novas have the 8.5" 1976-79 Novas could have an 8.5" or a 7.5" 10-bolt.


Yes, aftermarket 30-spline spider gears are available for the 8.5" 10-bolt but I would rather install aftermarket 30-spline axles instead of stock 12-bolt axles.


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We do this any time someone orders new axles and a posi unit ath the same time since it's all the same price. at least 2 times a week weather its an early 8.5 camaro or just a 28 spline p/u truck. We do it all the time and if you call any diff shop or distributor they will recomend the same thing. But good thinking on your part if you did all the figuring on your own, most people just buy standard replacement parts.

Miles:D
 

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Discussion Starter #6
Thanks

What sinched it was a call to (sorry) Randy's ring and Pinion 2 years ago. After talking to one of the machinest there. He told me that all the 30 spline used the same broach and all 28 spline used broach. then he went on to say that when they forge axles they are all the same batch just the broaching is different! 10 bolt and 12 bolt axles are the same lenth just different broach and all gm 30 spline used the same broach just like all the gm 28 spline.

just different by rear end lenth or application.

JBimmolate said:
We do this any time someone orders new axles and a posi unit ath the same time since it's all the same price. at least 2 times a week weather its an early 8.5 camaro or just a 28 spline p/u truck. We do it all the time and if you call any diff shop or distributor they will recomend the same thing. But good thinking on your part if you did all the figuring on your own, most people just buy standard replacement parts.

Miles:D


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I'm sure I'm confused, but in order to get a 12 bolt in an 8.5" 10 bolt housing, doesn't the size of the ring gear need to be changed, along with the carrier, not just the axels?

I've got my 8.5 10 bolt built pretty stout, but if there's a way to get a bigger ring gear in there, I'm all for it. I just can't imagine being able to do that without some major machine/fabrication work.

Good info though!
 

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I had to read this several times and I think the title is misleading.
You are not installing a 12 bolt ring and pinion into a 10 bolt housing.
The only thing you are doing is making a 28 spline ten bolt into a 30 spline ten bolt. The axle spline count has nothing to do with the number of bolts holding the ring gear.
 

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Discussion Starter #10 (Edited)
OK For clarity: up grade to 30 spline.

I am sorry I created so much confusion. I should have started out by saying that the 8.5 and 8.875 rearend share many of the same features, like axle lenth, pinion diameter,pinion bearing diameter, and spline count if using the correct year carrier.

When consering if the 8.5 would hold up to 600+hp the 8.5 should not be discounted. NHRA certs the 8.5 to the 8.40s in the 1/4 when built correctly. Detroit has the 8.5 30 spline Trutrac running 8.20s in the 1/4.

The difference between the 8.5 and the 8.875 is only .1875" on the radius or .375" on the diameter. Most of the Breakage you hear about in the 8.5 is the "A" taper axles and its low spline count or twisted splines.

I figured out a way to install a 12 bolt 30 spline axle in to a 8.5 housing. When you consider that to only differance between the 8.5 and the 8.875 after a 30 spline upgrade to the 8.5 is only the diamiter of the ring gear and bolt count... but all else is the same. you make the call. I applogise.



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Moser Engineering has been doing this for a while. I think they also have kits for puting a 33 spline axle in a 12-bolt housing.
 

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Old thread I saw while posting somewhere else. Wanted to add to this one for anyone doing a search. Around 2003 GM changed the ring gear of the 8.5 to an 8.6. You can put the 8.6 gears into an 8.5 withotu issue. This closes the gap in diamter between 8.5 10 bolt and 8.75 12 bolt.

Someone mentioned if there was a way to install a bigger ring gear into a 10 bolt that they would do it. This is a solution to that request.

FIY
 

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Discussion Starter #16
yeah a while back.... ..I pieced this thing together.. ordering parts and such from various sources.... at the time.. I had no Idea anyone was assembling a "kit" to do this....


ended up talking to DTS, Randy's. Foot Axel (superior Axel parent co.) and many others.. in determining what bearing, Axel, carrier..etc.. I was very Excited when I put everything together and it all fit as planed...(hence the post)

that was 3 years 11 months ago.. and its still not leaking!:D
 
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