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Discussion Starter · #1 ·
I am considering a few 12 bolts for my 70 Chevelle. I know I can use any 12 bolt from a 68-72 GM A body. It will more than likely be open as posi is hard to find & a lot more $$.

My questions are these?
1) If I pull diff cover, how can I determine rear ratio? Counting teeth? Turning yoke & counting wheel revolutions? Stock vs aftermarket gears?

2) If I put white lithium grease on ring gear, and turn it what should the mesh look like?

3) Is there any way to check for bent axles other than pulling them? Same question for axle bearing/surfaces?

4) What other kinds of things should kill a deal on a rear?

5) What kind of range does an open 12 bolt go for where you are at?

6) Finally, anyone got a line on a yard in Central/Southern Ohio where I can find 12 bolt rears?



Premium Member
9,707 Posts
1) the ratio is marked (in code) on the axle tube and it is stamped on the gears
3) Bent axles may be checked with an indicator on the flange while the shaft is rotated.
4) Look for cracked or broken shock mounts; you may need to do some welding to fix them.
5)I got a 2.73 ratio for $125 and am willing to move it out for the same.

150 Posts
If you pull the cover, most gear ratios are stamped on the ring gear. Actually the gear tooth count is stamped there.
Example: 41 10
Take the 41, divide by 10 and you have 4.10:1 ratio. You most likely will come up with something other than these numbers, but you take the large number, divide by the small number and will come up with the ratio.

The second question about the mesh: After adding the grease, the ring gear should have a worn patch in the center of both sides of the teeth. Backlash should also be checked and should measure between 8-12 thousandths (.008 - .012)

The axle bearing surface can be checked by running your fingernail over the surface the bearing rides on (after pulling the pinion pin and c-clips) If you get a snag on your fingernail, the bearings are shot, axles can be reused if not bent by using axle saver bearings (not as good as stock type, but work ok)
BTW, heat up the carrier with a torch where the little bolt goes into the carrier and pinion pin before you try to wrench it off with the 5/16 wrench, or you will break it off in the housing. They have Locktite on them. They are very hard to get out if broken off unless you have the special tool to get them out.

The other thing I will tell you is this:
If you are buying a used rear, plan on replacing all the bearings, adding an Auburn posi and replacing axle bearings & seals. If the gear ratio is not to your liking, replace the ring & pinion as well. Have a reputable driveline shop set up the rear for you before you install it, and you will be set. Plan on either using a conversion u-joint, or having your driveshaft rear weld yoke changed to mate up to the 12 bolt if your car had a 10 bolt in it originally. Also plan on spending $1000-1200 for all these goodies.
Estimated costs as follows:
Auburn posi - $400-$500
Bearing/shim kit $100
Axle bearings/seals - $50
Labor to setup rearend - $100-$300
Cost for used open 12 bolt housing - $100
Gear set - $150
Conversion joint - $25
Driveshaft welded/balanced - $60
(it adds up quick, no?)

Sorry this is so long winded, but good luck in your search!

[This message has been edited by Scooter (edited 12-18-98).]

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Some providers are using cacheing technology to reduce their load. this prevents anyone on their system from seeing changed pages such as this. The other alternative is that your computer is set to 'not verify' pages once they are visited. The server is working fine, so I don't know what else to tell you.

I've emailed this file to you.

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