Help with IDing rear diff

[Dleslie212]

77 K10, trying to ID my rear diff. I can't find a plate on it anywhere and I want to make sure I can get a new gasket so I can change the fluid. Anybody recognize this? I'm pretty new to trucks, and don't know a whole lot yet. I Googled 12 bolt GM rear ends, but everything I've found says they weren't on a 77 K10

https://imgur.com/gallery/kFvn96T

[sweetk30]

12 bolt truck rear diff .

skip the gasket and get a tube of RIGHT STUFF gasket maker and use it to make a decent bead .

or do a gasket and some rtv on both sides .

the last few diff cover gaskets i did 100% dry and clean are ozzing fluid . so i need to drain and reseal dam it . going with option one i posted above like i should have from the start .

[Dleslie212]

Originally Posted by sweetk30

12 bolt truck rear diff .

skip the gasket and get a tube of RIGHT STUFF gasket maker and use it to make a decent bead .

or do a gasket and some rtv on both sides .

the last few diff cover gaskets i did 100% dry and clean are ozzing fluid . so i need to drain and reseal dam it . going with option one i posted above like i should have from the start .

Thanks for the advice. So do you think this is something that a previous owner had swapped in at some point? Any idea if it's a decent rear end?

[Dleslie212]

I ended up pulling off the cover. It looks like it maybe has limited slip or something but I'm not positive what it is exactly. Also there's a tiny little gear deal with some broken teeth and a shaft that's all stripped. Anybody know?

https://imgur.com/gallery/UyAfXzt

[Tom]

Originally Posted by Dleslie212

Thanks for the advice. So do you think this is something that a previous owner had swapped in at some point? Any idea if it's a decent rear end?

Its the oem rear on the trucks until the 80's then they switched to the 10 bolt.

[Dleslie212]

Originally Posted by Tom

Its the oem rear on the trucks until the 80's then they switched to the 10 bolt.

Thank you

[hatzie]

Eaton G80 Gov Loc and it's on the way out.

[Dleslie212]

Originally Posted by hatzie

Eaton G80 Gov Loc and it's on the way out.

Just my luck. Do you think this is something worth trying to repair? Or should I replace the entire carrier? I'm thinking about throwing the cover back on, filling it with fluid, and forgetting about it untill I can buy myself a new one for Christmas

[hatzie]

They have a bad reputation for doing just exactly what this one did and then dropping the weights into the ring and pinion.
Right now you don't necessarily need ring and pinion gears.
If you're careful and lucky it'll last til you can replace it. If you don't luck out those weights will get into the gears and make a larger mess.

Get a Limited Slip Diff. It'll behave and it won't explode.

[Dleslie212]

Originally Posted by hatzie

They have a bad reputation for doing just exactly what this one did and then dropping the weights into the ring and pinion.
Right now you don't necessarily need ring and pinion gears.
If you're careful and lucky it'll last til you can replace it. If you don't luck out those weights will get into the gears and make a larger mess.

Get a Limited Slip Diff. It'll behave and it won't explode.

I just read that those weights can be removed and basically turn it into an open diff, so that's probably what I'll do to buy myself some time.

What are your thoughts on the Eaton TrueTrac?

[hatzie]

The Yukon clutch pak LSD will react faster and more consistently. And they are less expensive.

[Dleslie212]

Originally Posted by hatzie

The Yukon clutch pak LSD will react faster and more consistently. And they are less expensive.

What would I need to know when shopping for one of these? What housing I have, and how many axle splines?

[hatzie]

The 12bolt truck axle is 8.875" ring gear and the axles are 30 spline.
The 12bolt car axles could have several spline counts but the 12bolt truck used 30 spline.

There are three different carrier sizes. You'll need to know what the ring & pinion ratio is...

• 2.73:1
• 2.76:1 to 3.42:1
• 3.73:1 & higher numericial ratios.

The differing carriers ring gear mating surface is offset for the thickness of the ring gear. The offset places the ring gear teeth in the correct position to mesh with the pinion gear.

[Dleslie212]

Originally Posted by hatzie

The 12bolt truck axle is 8.875" ring gear and the axles are 30 spline.
The 12bolt car axles could have several spline counts but the 12bolt truck used 30 spline.

There are three different carrier sizes. You'll need to know what the ring & pinion ratio is...

• 2.73:1
• 2.76:1 to 3.42:1
• 3.73:1 & higher numericial ratios.

The differing carriers ring gear mating surface is offset for the thickness of the ring gear. The offset places the ring gear teeth in the correct position to mesh with the pinion gear.

Thank you

[Matt Man]

I have a Truetrac in the front 10 bolt on my 81 and its been flawless no issues. The only problem is for the 12 bolt (and 10 bolt), you need the 3.73 and up carrier to use.

[Dleslie212]

Originally Posted by Matt Man

I have a Truetrac in the front 10 bolt on my 81 and its been flawless no issues. The only problem is for the 12 bolt (and 10 bolt), you need the 3.73 and up carrier to use.

I thought ihe truetrac was a straight carrier replacement. Did you mean the ring gear? I don't know alot about these things sorry

[hatzie]

Originally Posted by Dleslie212

I thought ihe truetrac was a straight carrier replacement. Did you mean the ring gear? I don't know alot about these things sorry

It is a straight carrier replacement.
As I noted a couple posts ago there are three carrier sizes.
Eaton doesn't cover the 2.73:1 carrier and I don't see any others listed for that size either. If you have that gearset you're changing gear ratio or not putting in an LSD.


You do have a choice of using an Auburn or Yukon with your rear end... not just Eaton. I've run Auburns in 3.75" Chrysler rears under a 69 Fury and two 69 Dodge Polaras. All three had built B383 engines and A833 transmissions. I was not gentle with any of them.
The Auburn LSDs cured my one wheel peels and hooked up both sides near instantly under some challenging conditions and they did it without exploding. I turned propshafts into pretzels and broke ears off yokes but the rears held up just fine.
https://www.justdifferentials.com/category-s/2285.htm

[hatzie]

Usually the gear ratio or at least the tooth count and part number is stamped on the side of the ring gear.
Just spin it around til the stamp is showing.

[Dleslie212]

Originally Posted by hatzie

Usually the gear ratio or at least the tooth count and part number is stamped on the side of the ring gear.
Just spin it around til the stamp is showing.

Here are some pics of the stamping I found. It's hard to read the numbers - I can't really tell what I'm looking at. Also, there's some pretty bad hashes on the ring gear - does this require replacement?


Ring gear stamping https://imgur.com/gallery/ik9wO7n

[sweetk30]

ring gear looks like 1978 made .

as to ratio that got eaten up when they cut the gear . . looks like your counting .

but if i was to make a crazy shot at it from your pic and guess maybe 41/11 = 3.73 and they were common .

[Dleslie212]

Originally Posted by sweetk30

ring gear looks like 1978 made .

as to ratio that got eaten up when they cut the gear . . looks like your counting .

but if i was to make a crazy shot at it from your pic and guess maybe 41/11 = 3.73 and they were common .

What do I go about counting? How many teeth on the ring gear?

Or should I just plan on replacing ring and pinion as well due to those chunks missing?

[hatzie]

Originally Posted by Dleslie212

What do I go about counting? How many teeth on the ring gear?

Or should I just plan on replacing ring and pinion as well due to those chunks missing?

If they're damaged replace them. No reason to load a new carrier with chewed up gears.

You probably get to install bearings and seals in the end of the axle tubes as well.

I'd look into what it would cost for a used 10 bolt or 12 bolt with the same ratio and a warranty. Pull the cover and look it over before you take it home.

[hatzie]

Mismatched front to rear axle ratios can and probably will do bad things to the transfer case and can foul up the transmission and/or one or both of the axles as well.

If you haven't put it in 4wd without a catastrophe you probably want to verify the front ratio.

3.73 is an optional 1/2ton axle ratio. Your SPID label in the glovebox should call out an RPO code for the axle ratio installed at the factory. GT4 is 3.73 axle gears and G80 is the Eaton Gov-Loc differential.
I'd be fairly confident the axles haven't been fooled with if the SPID matches the parts installed in your rear axle... but verification never hurts. You have to pull the axle cover to change the gear oil.

The G80 does bad things when used with limited slip additives so someone that knows will likely look at the SPID and assume the Eaton is still in place. If you put a limited slip diff in the axle it's a good idea to put some kind of tag on an axle cover bolt above the axle tube noting that Limited Slip Additive is now required.
This is one of those "posi" tags from GM.
https://www.ebay.com/i/330781133705?...d=330781133705

[Dleslie212]

The transfer case is a whole other thing... I took it out a few weeks ago and tried to pop into 4WD, but the linkage was seized which is a pretty common thing I guess. Anyway, linkage works now, but I haven’t had a chance to take it out and test it. It is a NP203 that I am 50% sure has been converted to part time. I’m going to jack it up tonight and see if I can spin the front driveshaft while it’s in gear. I’ll also look for that tag in the glove box, I wasn’t sure where the tag was supposed to be. And I’m sure I’ll end up opening the front diff at some point this weekend to take a look in there as well