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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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