I'm kind of a GM noob when it comes to diffs, but here goes. Climbed under the Vista this after noon and found a 12 bolt under the car.

What I need to know is, did they make peg-legged 12 bolts? Is there a tag I need to look for on the pumpkin to see if it's a posi unit? I would be the cat's *** if it is a posi.

 

If it's the original rear end it is a "12 bolt" housing with an 8.5" ring gear. It is known as an "O" rear end. More than likely it is not a posi unit. If memory serves me, you could find out by lifting the back wheels off of the ground and spinning one wheel as if you were driving forward. If they both go the same way it is a posi rear end. If they turn opposite ways it is an open rear end.

Steve

 

Not to let the wind out of your sail but that 12-bolt cover has a 10-bolt ring gear inside unless it was built in Canada in which case it would have a Chevy 12-bolt. If the cover is smooth, it is the O-type.

 

You can just jack one side - if it spins, it's open!

 

Well that's ok, would have been icing on the cake if it was a posi unit.

 

WoofWgogon: You can try to look from behind. On the right side near the right side middle of the axle, there should be a two letter code there. Let us know if you see two letters there. I hope that this can help.

 

I'm a pretty big dude so it may be a bit before I can back underneath car.

 

You could get the 12-bolt with or without a posi.Posi was something that needed to be ordered,or checked-off in the option box.

 

I'm going to have the jacked up here probably on Saturday to bleed the brakes, I'll check the rear for wheel rotation to see if it's a peg leg or not.

 

Hey Brian,

So after checking my differential, i also jacked a wheel and it spun. Hence, i confirmed it's an open model. Also wirewheeled the tube and found out its 2.73:1

Do you guys know where the fill hole is on my diff?? I would like to change my oil, probably with standard 80w90 gear oil. To drain i guess i need to remove the cover? But how do i fill it up?

 

If you block the wheels, and put it in neutral, mark both the driveshaft and the tire.
Counting the revolutions of the driveshaft per one revolution of the tire, will let you approximate your rear end ratio.

 

Tony,
You would need to remove the cover to drain it.The is a plug on the passenger side of the pumpkin,in between the ribs,and that is what you remove to fill with fluid.

 

Awesome! Thanks!

No wonder why i couldn't find it, i was under there yesterday looking at the driver side. Do i need any special tools to remove it or is it just a socket?

 

Just a 3/8 drive ratchet or breaker bar.The plug has a square female opening,for the ratchet to fit into.Clean it out real good,so the ratchet engages all the way.

 

Okay Cool, Thanks i'll try that.

I've got a couple quarts of 80W90 gear oil in my garage. Should i be worried if it's synthetic or not (it might be synthetic, i bought it along time ago)? Not sure what's better for my open differential?

 

The synthetic will be fine for a non-posi rear,but do NOT use it with a posi.

 

It's an open diff, had the car jacked up to do brakes and spun the rears. No posi .

 

If you're removing the diff cover to change the fluid it'll be worth a quick look at the carrier. Pretty easy to tell if it's a posi unit inside vs the open diff. The ratio will also be stamped on the ring gear. If it is 2.73:1 it'll be something like 15-41 stamped on the edge of the ring gear.

 

yep ratios will be the following for the 12 bolt olds rear
16-41 2.56
14-39 2.78
13-40 3.08
13-42 3.23
12-41 3.42

there is also 3.91, 4.33 4.66 5.00 doubt it will be any of those....
Jim