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Can you identify this pic?

3K views 8 replies 5 participants last post by  leeklm 
#1 ·
After searching the topic, the last thing the site needed was another rear diff thread, but...

I thought that I had a chevy 12 bolt rear diff, but was recently told it is actually an Oldsmobile rear diff, which has 12 bolts, but is an 8.2" rear.

Can anyone confirm this based on the attached picture? I really want to get rid of the 3.9 gears and install 3.36 or 3.55, but now am not sure what carrier I would need to purchase.

Talked to a local guy that has an OEM posi carrier with 3.08 gears for $600, but i would prefer new at that price.

Thoughts? Thanks!
 

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#3 ·
The "O" type 12 bolts have a almost round cover and are hard to find parts for and are expensive when you do. Almost bought one advertised as chevy 12 bolt. As you said being you have a 4 series carrier you would need to find a 3 series for the gear you want.With the grand its gonna cost for parts and labor you would be better off looking for a chevy 12 bolt or a Pontiac 10 bolt LSD complete with 3 series gear and carrier.
 
#4 ·
I was afraid of that :( The shiny red paint on the car must have dulled my eyesight. Guess I will know for next time...

I assume all late 60's A body rear ends will work for the 66? What others would be a direct "bolt in"

Thanks again for the feedback, it is appreciated.
 
#5 ·
yes any of the A-body (coil spring) gm rear ends will work 68' and later are 1/2" wider plate to plate but will still work. You can also buy Ford 9" rears with GM perches for the A-body from moser and others which are pretty much bulletproof. The ford 9" and chevy 12 bolt Car dif's are the strongest for high performance the BOP 10 bolt is just fine for a driver. The Chevy 12 bolt cover has an eyebrow stamped in it for identification.

http://novaresource.org/images/rears.jpg
 
#6 ·
In my 1967 GTO with Muncie M-20 four-speed, I went from 3.90:1 to 3:55:1 gears and got virtually no reduction in RPMs at speeds over 60 MPH. I decided to install a Keistler five-speed manual and the car runs at about 1,900 RPMs at speeds in excess of 60 MPH. My 2 cents -- spend the money on a transmission with overdrive instead of spending on the rear gears. Good luck with whatever course you decide to take.
 
#7 ·
Good point on the Trans, but I would also like to have a posi carrier as well at some point in the future, thus still needing the rear. My current 400 smogger does not really need more than one wheel, but that WILL change next winter... Checked with Summit today, and a set of Richmond 3.42 gears is $300 for this axle, and no available posi carriers. Ouch! Guess it is time to start the "rear-end" search.
 
#9 ·
From a car craft article...

From '68 to '70, Oldsmobile used an axlehousing with a 12-bolt rear cover, but internally, it employed an 8.5-inch-diameter ring gear with 10 attaching bolts. You mentioned that you were looking through your 12-bolt Olds housings for a 12-bolt carrier; you can stop looking because you won't find one-they're all 10-bolts. This is still an 8.5-inch-diameter carrier, so it is stronger than the 8.2-inch 10-bolts, but it is somewhat rare, as its internal parts are not the same as other "corporate" 8.5-inch housings used in later GM models. Richmond Gear does make 3.42, 3.90, and 4.10 gearsets for this Olds-only (not BOP) axle assembly, and according to Richmond's catalog, those gears will fit a 2.93 through 3.23 ratio differential. The basic rule of thumb is that a 2-series differential will only work with gear ratios in the 2 series, as in 2.56 or 2.78. The other ratio series follow a similar path. This is not a hard-and-fast rule, however. For example, one Buick/Pontiac 8.2-inch rearend differential will mount 2.87 through 3.31 gears, while another will accommodate 3.36 and deeper ratios well into the 4s. So you need to be careful here when matching a differential with a given ring-and-pinion. Just to make it more confusing, many aftermarket gear companies offer, for example, a 4.10 gear for a 12-bolt Chevy housing that will mount to a 4-series carrier as you might expect. But some companies also make a 4.10 gear that will mount on a 3.08 through 3.73 (3-series) differential by making the ring gear thicker.

Read more: 10 Bolt BOP - What's Your Problem? - Classic Muscle Car - Car Craft Magazine
 
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