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i am recently researching about differentials in hope i can make it posi and i was wondering how do you measure to know if the ring and transfer case is 7.5 or 8.5 and what is the difference between those besides the size does one give more power than the other does one is one better for highway or drag if someone could please give me some insight about this it would be very much appreciated thank you
 

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i am recently researching about differentials in hope i can make it posi and i was wondering how do you measure to know if the ring and transfer case is 7.5 or 8.5 and what is the difference between those besides the size does one give more power than the other does one is one better for highway or drag if someone could please give me some insight about this it would be very much appreciated thank you

Being simplistic, 8.5 is OK, 7.5 not so much
The 8.5 would be better in almost any application
 

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72 Frame off, bare metal resto-mod. 383, TH350, Eaton Posi, Complete new suspension, disk brakes
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Get out the wire brush

There are casting numbers on the side of the center section of the rear axle. Often called the "pumpkin" by gearheads, there are casting numbers that will tell you size, casting date and where the casting was done.
Here's a link to the codes and more than you probably want to know. http://www.novaresource.org/axle.htm
While a "12 bolt" is often the GM stronger axle - 12 bolts holding the rear cover on the pumpkin - there are a lot of folks running the "10 bolt" 8.2 and 8.5 rear axles. The big conversion is to use a Ford "9 inch". While parts are available, I don't recommend it for a first timer. Within the 10 bolts there are two different axles. Most will have 28 splines at the inner end, there are some heavy duty axles that have 32 splines. You will need to know that number if you are going to use your current axles as I did. It depends on what you plan to do with the car.
In my case, I changed out the factory open carrier for an Eaton Posi and installed Richmond 3.73 gears; of course all new bearings, shims. It took me a bunch of hours and a lot of fitting. There's a lot of information on this website: http://www.ringpinion.com/ Their shop happens to be about 4 miles from my house, so that made it easy for me. If you choose not to set the gears up yourself, most 4x4 shops do it, but may want to do the whole job, not just set up the gears.
Now the next obvious question, I spent about a thousand bucks by the time I was done. There are pictures of mine being setup on my build link below.
Let us know what you decide and keep us up to date.
 

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ok thanks guys see what i just want to do is keep it original for now and it was posi before but during a trip the whole brake drum wheel and tire came off and i had to change the whole differential to one from a buick skylark not posi, i did take the gear on the inside of the other differential out but apparently it isnt a posi unit so now im in the process of making it posi. i want to make it a gear ratio of 3.42 since i use the car a lot and that one is still good for distant travels and still has power to it i have a 350 it has a performer intake and 750 cfm 4 barrel carb headers and dual exaust and i was told that was the best without having to making major changes to the engine. and how do you measure if it is 7.5 or 8.5 do you measure the inside of the ring gear or the outside
 

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well if I had a link to the pictures you can see the difference in differentials by looking at the rear cover/ housing. The 8.5 chevy has a minor difference in housing to the buick /olds /pontiac rear... but bolts up the same way. The 7.5 rear is a different animal all together. I have seen 7.5 hold up to some minor abuse.. but the money is better spent on a 8.5 10 bolt. I have seen 8.5 10 bolts in sub 10 second cars make hundreds of full throttle pulls at the track and have no issues... but it needs to be built right.

theres links all over that shows rear diff covers differences. If money is a issue, stay with that 7.5 as long as it works and drive it especially if it isnt a track car. Save the bucks and get a 10 bolt 8.5 under there when it blows. If your crazy serious about making it bulletproof... youll need to find either a 67 - 69 camaro 12 bolt.. or get a custom 9" ford rear.

10 bolt 8.5 is MORE than enough for the street and weekends at the track, just keep it under 500 hp okay:D
 

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Thanks

ok thanks guys that really helps a lot and yeah im going to save for the 8.5 cuz my way of thinking is that if im going to spend money on it might as well add a bit more power to it :D
 
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