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  #16 (permalink)  
Old 01-25-2013, 02:26 PM
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Originally Posted by CHEVILLAC View Post
Ok, I've been doing a ton of reading and have come to the conclusion that a '79 8.5" 10 bolt with 28 spline axles is stronger than a '73 8.75" 12 bolt truck with 28 spline axles because the pinion shaft diameter on the 10 bolt is bigger than the 12 bolt. The only other difference I can find between the two besides ring gear diameter and track width is parts availability and interchangability. Parts for the 10 bolt are more interchangable as well as cheaper and easier to find, especially in a posi application. So when I can save up some money and get a 3 series posi carrier and 3.73 gear set for the 10 bolt, I'll adapt it to fit under my truck. I have trailing arm/airbag rear suspension so getting the 10 bolt to mount up will be a breeze. I don't mind the narrower track width since I'm going to run cheater slicks, that means I can get wider tires. In the meantime I just have to hope the 3.08 open rear will hold up. You guys think it will hold up to 400hp or so, or should I go easy on the pedal (which won't be easy)?

I don't know where you got that information, but it's a bunch of crap. First of all, NO 12 bolt had 28 spline axles. They ALL had 30 spline axles, except the very early truck 12 bolt rear ends that had the coarse splines. Your 12 bolt has 30 spline axles.

Next, the 8.875 ring gear in your 12 bolt is MUCH stronger than the 8.5 inch ring gear in the 10 bolt. The pinion shaft is not a problem either. Both have a 30 spline pinion shaft, the only difference is the truck 12 bolt has a smaller inner pinion bearing. This is not a problem. I have never heard of anyone breaking a truck 12 bolt pinion shaft or having a problem with the inner pinion bearing. 9 inch Ford rear ends have a very small 28 spline pinion shaft, and everybody is always talking about what a great rear end the 9 inch Ford is.

Yes, parts for the 8.5 are usually easier to find, but the truck 8.5 limited slip differentails are all the Governor Lock. This is one of the worst differentals you can use in a high performance application. The only thing that they are known for is how quickly and easily they break.

On to the differential bearings. The truck 12 bolt has larger differential bearings than the 8.5 10 bolt. This means that the differential is going to be held in place a little better in the 12 bolt

All together the 8.5 has nothing better than the 12 bolt except the inner pinion bearing. In all other areas the truck 12 bolt is much better. Your best way to go is to use the 12 bolt that you have and install the correct parts in it to make it do what you want it to do.

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  #17 (permalink)  
Old 01-25-2013, 02:54 PM
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This is getting frustrating. So there is no LS diff carrier for 12 bolt truck rear ends that are NOT gov lock? Unless I go aftermarket and pay through the nose. I can't afford to put $700-$1000 into the differential, $300-$400 maybe, that's why I was opting for thr 10 bolt. I paid $200 for the 10 bolt, not knowing I couldn't put better performance ratio gears on a 2 series carrier. I know they make a spacer but I've heard some really nasty tales surrounding them. I drive my truck hard so I need it to hold up. Is it possible to have a 3.73 gear made that will fit the 2 series carrier without a spacer? As many times as I've seen this issue presented, I would think Richmond or Auburn or SOMEBODY would make gear sets for this application. Which ever route I go, it will be a while because I have to save up money. In the meantime, will the 3.08 open 12 bolt get me by? Should I nurse it or do I not have anything to worry about but lack of traction?
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Old 01-25-2013, 03:03 PM
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The 12 bolt is going to hold up fine. It's only going to spin one wheel, but it's not going to break. If you do a lot of one wheel burnouts you will burn up the differential gears and the cross shaft, so I dont recommend it.

The truck 12 bolt did come from the factory with a Eaton Posi in the early '70s, but the case on those was weak and would crack. The new Eaton Posi for the truck 12 bolt is very strong and I highly recommend it. Tom's Differentials probably has the best prices on these, so contact them for the parts you need.

There are no thick gears for the 8.5 10 bolt to allow you to use the 2 series differential. The only option that you have with the 2 series is to use a spacer. I do not recommend that. I have seen the results of using ring gear spacers. Some people get away with it and others don't.

All of the 8.5 truck rear ends that had the limited slip differentail had the Gov Lock. Only the car 8.5 got the Traction Lock differential.
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  #19 (permalink)  
Old 01-25-2013, 03:14 PM
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Originally Posted by big gear head View Post
The 12 bolt is going to hold up fine. It's only going to spin one wheel, but it's not going to break. If you do a lot of one wheel burnouts you will burn up the differential gears and the cross shaft, so I dont recommend it.

The truck 12 bolt did come from the factory with a Eaton Posi in the early '70s, but the case on those was weak and would crack. The new Eaton Posi for the truck 12 bolt is very strong and I highly recommend it. Tom's Differentials probably has the best prices on these, so contact them for the parts you need.

There are no thick gears for the 8.5 10 bolt to allow you to use the 2 series differential. The only option that you have with the 2 series is to use a spacer. I do not recommend that. I have seen the results of using ring gear spacers. Some people get away with it and others don't.

All of the 8.5 truck rear ends that had the limited slip differentail had the Gov Lock. Only the car 8.5 got the Traction Lock differential.
You definitely seem to know your stuff so I'll get in touch with Tom's and see what kind of money we're talking about. I'll also have to pay a local shop to set everything up for me too because I hear it can be tricky and is best left to the trained eye. I'm glad to know the way it's set up now will get me by, I won't be too hard on it. I have to get this motor in my truck asap because I don't think the one that's in my truck now is going to last much longer.
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