CowsFlyTogether

I'm in the market for a posi diff for my '70 Cutlass Supreme. Currently, I have the factory 2.54 open diff behind factory TH350. I'm hoping to bump up the gear ratio just a little bit (maybe 2.73 or 3.08. Possibly 3.42 max). Application will be just street; I just want a little more pop off the line and that classic muscle car two-line streak on the pavement

I recently discovered that rockauto has Eaton posi units for a relatively affordable price. Anybody have one of these or heard of any good/bad reviews? What other options have other people used for swapping in a posi unit? I've checked other places (Richmond, Strange, etc) and they seem a little expensive, but are known for good quality.

I've spent months on craigslist and the for sale forums here on CO and really haven't found anything that fits what I'm looking for. I've been burned too many times checking out craigslist posts and what the seller advertised as good condition be total junk or way too expensive.

Thanks in advance!

Fun71

Your '70 should have come with an O-Type 12 bolt rear. If that is what you have, the choices for differentials are very limited and the typical stuff you see in the catalogs won't work in it. You would need a carrier specifically for an O-Type 12 bolt and not one for a Chevy 12 bolt or an 8.5" 10 bolt.

So verify what rear you have under your car and then go from there.

CowsFlyTogether

It's a 12 bolt cover, 10 bolt ring gear. So unless I'm mistaken, it is an o-type

joe_padavano

Which means that there are NO aftermarket parts available for it from RockAuto or any other vendor.

CowsFlyTogether

Bummer, I guess I'll have to hunt around for an anti slip from another cutlass. Thanks for the response!

ELY442

You can switch it over to a Chevy 12 bolt rear, if you like.

brownbomber77

Something to ponder even though he wants $200 for it.... 3.23 gears for O-type

http://chicago.craigslist.org/nwc/pts/4650949433.html

rookieoldsguy72

I bought a used 12 bolt posi-unit at Dick Miller races held at Summit Racetrack in Norwalk Ohio from Webb Racing.
I would sell for what I payed for it has 2.73 gears. I'm was told won't work in my 1972
$400.00.
Rookieoldsguy72

Fun71

Check with monzaz or 507olds to see what they have available - they are our resident differential gurus.


This is an option since you said you wanted to maybe go to 2.78 gears. Be aware that the same carrier is used for the 2.56 and 2.78 ratios so this one wouldn't allow for installing lower gears.

FYI, here is some O-Type info I got from Brian Trick many years ago. This might help you in your search.

Here is a list of the posi units & what gears they will fit:
#673 = 2.56 & 2.78 ratios, 28 spline
#671 = 3.08 & 3.23 ratios, 28 spline
#588 = 3.42 & anything numerically higher, 28 spline
#672 = 3.42 & anything numerically higher, 31 spline
***The 31 spline axles were offered in late 67-68 OAI cars only.

CowsFlyTogether

Thanks Fun71! That's really helpful info to know.

rookieoldsguy72: I'll send a PM if I decide to take you up on the unit you have for sale. Sounds like exactly what I'm looking for.

joe_padavano

If your 72 has the stock rear axle, that would be corporate 10 bolt axle, so no, it won't work. Is your "12 bolt" posi for a Chevy 12 bolt (8.875" ring gear) or for a Type O axle (12 bolt cover, 10 bolt 8.5" ring gear)?

rookieoldsguy72

Morning Joe,
Monzaz Installed a 3.42 gear and posi unit in the stock corporate 10 bolt axle already under car. He informed me that was the most cost effective way to go vs using the Type 0 axle(12 bolt cover 10 bolt 8.5" ring gear) 2 yrs ago when gears were an issue along with Driveshaft lenth which worked out good for us as car is 90% original Great Grandmothers car.
(Have dealt with Jim/Monzaz & Brian Trick both great guys.