Yes...posi or not posi?

[Siedl]

I've been reading through previous threads regarding g identifying posi, locked or open. Reading so many different explanations there are discrepancies and I need to be clear on the rearend I have.

I am pretty sure it's posi but could be locked...

Jacked it up. Spun the one tire; wheels move in opposite direction. Spun the pinion yoke and both wheels turn the same direction. (That's my clue it's posi) but when I take the cover off there's no spring in the middle and I don't see or can't identify the clutch plates.

1. Am I correct that spinning the WHEEL does NOT determine posi, or not?
2. If spinning the YOKE both wheels turn the same direction AND there is no spring under the cover is it for sure locked?
3. I have the number from the ring gear but can't find the decoder for it.
40 13 gm c (5or6) z11 721

Thanks so much! Love you guys!

[Stocker]

You spun a tire one way, and the opposite side tire spun in the opposite direction. It is an open diff. If it had a posi or was locked, both tires would spin in the same direction. Ignore the yoke test, it's not valid here.

[leddzepp]

You have a 3.07 open diff

[truckster]

To decode the ring and pinion combination: 40/13=3.07.

[68Stepbed]

Does the carrier have some weird springs and counterweight on a shaft? If so, it has a Gov-Lock(posi). If all you see is four spider gears, it's an open(non-posi) differential. Also keep in mind the Gov-lock carriers can be misleading with the wheel turn test. It acts as an open diff until too much centrifugal force(one wheel spins too fast compared to the other) at which point the "locker" engages. This is also the reason they're referred to as Gov-bombs. They can tend to stay unlocked until one wheel is spinning a bit too fast and then decide to lock, causing catastrophic damage to the carrier and/or axles.

I learned from a dirt track guy that there's a way to modify Gov-locks to work like a LSD(Limited Slip Differential). I did it once and it actually worked quite well, but money is better spent on a quality name brand LSD or locker. I personally prefer the Eaton LSD.

Attached is a photo of a Gov-lock.

[Bigdav160]

The gov-loc didn't come out until 1973. So it wouldn't have been the OE unit for these trucks.

[Siedl]

Thank you for responding so quickly. If I may ask, why is the yoke text not applicable here? When would it be applicable?

So, I have an open differential and just to confirm for sure for sure, here is a pic.

Thanks again for all your responses.

[88fordf150]

Yep! open.

[randy500]

You also have an oldsmobile rearend if I remember right from the cover shape.

[randy500]

I looked it up, 8.2 inch ring gear BOP style.

[BILT4ME]

Yes, you have an open differential.

The Yoke turning does not apply because either one wheel will turn or both will turn or one will turn faster and one slower than the other, ASSUMING both wheels are off the ground. The yoke turns the pinion and the wheels will turn based on the RESISTANCE in the bearings, brakes, or tire friction. If you HOLD one tire, THEN turn the yoke and you cannot turn the yoke, THEN you MAY have a limited slip (actual POSI traction units are few....most are limited slip)

The MOST sure way without pulling a cover is to try to spin one wheel.
***If the yoke turns, and the other wheel does not spin, you have an open differential.
***If the yoke doesn't turn AND opposite wheel spins the opposite direction, you have an open differential.
***If the yoke doesn't turn AND the other wheel doesn't spin, you MAY have a limited slip.
***If the yoke turns AND the opposite wheel spins the SAME direction, you MAY have a limited slip.

Clear as mud?

Then you get into the differences between locking differentials VS limited slip. That's another thread. If you want to understand the differences, check out the website for Eaton Differentials and compare Detroit Locker, Eaton Limited Slip, True Trac, Lunchbox Lockers, Trac-Lok, ARB Air Lockers, Harrop E-Lockers, and others.

[GASoline71]

Originally Posted by randy500

You also have an oldsmobile rearend if I remember right from the cover shape.

Originally Posted by randy500

I looked it up, 8.2 inch ring gear BOP style.

Yup... looks like it is a BOP.

Here is a truck 12 bolt with a stock posi-traction unit in it.

http://www.67-72chevytrucks.com/vboa...&postcount=125

Gary

[Mike C]

[QUOTE=BILT4ME;7894649}
a limited slip (actual POSI traction units are few....most are limited slip)[QUOTE]

Posi-Traction is GM's Trademarked name for their Eaton sourced limited slip. Therefore "Posi" equals "limited slip". They are the same thing.

The Eaton "No-Spin" or later Detroit Locker is a mechanically locking differential which is a completely different animal.

[Siedl]

I'd like to thank the academies that brought us the engineers to invent the internet so that I could be "in the same room" as this great collective of gm knowledge. Thank you all very much! This is so much more efficient than battling poor reading comprehension.

Randy500; thank you for looking it up! Where did you look it up, I'd like to add as much of this to my research arsenal as possible. :-)


Thank you for the pic of the 12 bolt that was very helpful.

Thanks Bilt4me; definitely clear as mud :-)

Thanks Mike that's what I thought.

Have a great day! Thanks again!

[randy500]

I just know those old rearends and verified it on google

[hamjet]

Originally Posted by randy500

I looked it up, 8.2 inch ring gear BOP style.

Interesting, wonder why someone put an Olds rear in it. are they the same dimensions or was it modified to fit....

[Bigdav160]

Just an FYI, Oldsmobile axles are held in with a press bearing/collar. To remove you have to remove the brake backing plate. I don't know about Buick and Pontiac rear ends but Chevrolet would have had c-clip axles.

[Dustytrix]

[QUOTE=68Stepbed;7894274]Does the carrier have some weird springs and counterweight on a shaft? If so, it has a Gov-Lock(posi). If all you see is four spider gears, it's an open(non-posi) differential. Also keep in mind the Gov-lock carriers can be misleading with the wheel turn test. It acts as an open diff until too much centrifugal force(one wheel spins too fast compared to the other) at which point the "locker" engages. This is also the reason they're referred to as Gov-bombs. They can tend to stay unlocked until one wheel is spinning a bit too fast and then decide to lock, causing catastrophic damage to the carrier and/or axles.

I learned from a dirt track guy that there's a way to modify Gov-locks to work like a LSD(Limited Slip Differential). I did it once and it actually worked quite well, but money is better spent on a quality name brand LSD or locker. I personally prefer the Eaton

(Can you explain how you made it work like a limited slip?)

[68Stepbed]

[quote=Dustytrix;7896526]

Originally Posted by 68Stepbed

Does the carrier have some weird springs and counterweight on a shaft? If so, it has a Gov-Lock(posi). If all you see is four spider gears, it's an open(non-posi) differential. Also keep in mind the Gov-lock carriers can be misleading with the wheel turn test. It acts as an open diff until too much centrifugal force(one wheel spins too fast compared to the other) at which point the "locker" engages. This is also the reason they're referred to as Gov-bombs. They can tend to stay unlocked until one wheel is spinning a bit too fast and then decide to lock, causing catastrophic damage to the carrier and/or axles.

I learned from a dirt track guy that there's a way to modify Gov-locks to work like a LSD(Limited Slip Differential). I did it once and it actually worked quite well, but money is better spent on a quality name brand LSD or locker. I personally prefer the Eaton

(Can you explain how you made it work like a limited slip?)

Cut out the spring/centrifugal weight parts and shafts, and install an Eaton Preload spring and plate kit in between the axles.

I thought I had some pictures of what we did, but I can't find them.

[Dustytrix]

Thanks Matt I might give this a try. Gotta get engine swap finished first.