oldsonharmont

I found a posi carrier and 3:08 gears for a GM 10 bolt 8.5" differential. Seller says it is for the bolt in type axles, not c-clip. Will this work on my '71 SX open style differential with a 2:56? He claims they came from an early 70's Cutlass, and I can post pics of the set. Is there anything else to check on a deal like this?

Allan R

If it's from a GM 10 bolt, it's probably a 71/72 series 2 or 3 carrier. 3:08 gears are a nice compromise for performance and economy. If they are bolt in, they're likely from an Olds. Yes, this will work in your existing differential. You will probably need to have the carrier checked or rebuilt before you install it. There are 2 guys who specialize in this area: 507OLDS (Brian Trick) and Monzaz (Jim Mitschke). Send them a PM, they will have good answers for you about how to proceed. Have some pics to send along as well, its worth a 1000 words. Good luck with your changeover. BTW depending on which carrier you got, it might also work with your existing 2:56 gears. I know 2:56's are slugs off the line especially with a 350 but they are great for gas mileage on the highway. Just depends on how you want the car to perform.

507OLDS

make sure the unit is for 28-spline axles because that is what you have.You also want to find out what type of carrier it is.There are a lot of those Gov-Lock posi units floating around.Those came in trucks,but I would never put one of those in a car or performance application.

oldsonharmont

I took a pic of my diff, and the parts I found today. Maybe that will help more, thanks.
Seller says it is from an 8.5 GM 10 bolt with bolt in axles. The gear set is a 3:08.
The price is $80 for the gears and the carrier.

Allan R

Price is very reasonable. If this is for the carrier and gears that's a GREAT price. Are you planning to rebuild it yourself or have it done?

oldsonharmont

Right now I'd probably leave it to a pro, but anything's possible I guess. The price is great, but I want to be sure I'm buying the right parts. Being a '71 SX, I assume it's an open rear with a 2:56 gear. I'm looking for a bit more acceleration but still want to cruise it too. (if the weather ever cools off here!) The seller says it came off an early 70's Cutlass, but I'd like to verify it's the right stuff.

Allan R

The vast majority of these cars (probably over 90%) were built with open diffs. The pic of your diff doesn't show a posi tag so it's likely an open. You can tell for sure if you jack up both rear wheels and turn one. The other will go the other direction.
As far as gears, if you don't know the ratio you can find out easy enough while the car is up. Index mark a specific point on the driveshaft and tire with a white chalk (or something easy to see). Have a friend slowly turn the wheel exactly 10 times (using the index mark). You watch the drive shaft and count the number of revolutions it makes. If you have a 2:56 the driveshaft should go around 25 and a half times. If it goes around more than 25, take the total driveshaft turns and divide by 10. That's your ratio. The ratio is also stamped on the axle tube, but it can be really hard to find and uncover from all the grime.

507OLDS

$80.00 for all that is a steal.That IS the correct carrier for your rear.

Allan R

Hear that Oldsonharmont? One of the experts has given you the nod you were looking for. Run like the wind and buy that sucker!

oldsonharmont

Allan & Brian, Thanks for verifying the find, the guy at the swap meet said they were from an early 70's Cutlass, and he was hoping they would fit a later Chevy truck diff.
I also talked to my neighbor, who ran a speed shop for many years and he thought they might not be correct. But in his defense we were looking at pics on a digital camera, it was dark, and he didn't have his glasses on. I'll go with Brian and pick them up. I can always resell them if there's a problem. But $80 does sound great, and deals like that don't come along every day! Many thanks!

monzaz

MAke sure the casting on that posi is 725. That would be the 3 series. IF it is the 726 it will be a 2 series and the 3.08 gears will not work with out a spacer kit and the reason for the low low price.
So ask a few more questions and then go for it if it all pans out. Jim

JD

oldsonharmont

So tonight I call the guy with the parts for sale and find out he sold the posi unit later on Sunday. (but still has the 3:08's for sale for $20) Yes, I'm kicking myself now for playing the waiting game. (grrrrr!)

Allan R

I'd still pick up the 3:08's and put them in the car. You will see a BIG difference in the off line performance. Your revs will be higher, probably better torque band too, however your mileage will go down somewhat.

507OLDS

That looked like the 725 casting,judging by the positioning of the clutch notches & the ring gear flange.The other factory 3-series 8.5" posi carrier that they used during that era,looks pretty much just like that,but has the casting number 410408N,and usually has large 8's on it as well.