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bowtie_pasta

Need To Change Rear Diff Lube

Question

I checked the RPO code and I have the G80 rear end. Do I need to use an additive or can I just refill with 75W-90 synthetic? Also, how many quarts should I buy?

 

Do I need to buy a gasket or can I use RTV? Which RTV do you recommend?

 

Thanks GM Techs! :seeya:

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(disregard F-up ...)

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I believe the newer G80 doesn't use a traditional clutch pack, so additives are not needed. But don't take my word for it - I'm an old school mechanic going by memory ... which is dangerous practice. :seeya:

 

As far as RTV goes, I've been using Permatex Ultra Copper on everything for close to 20 years now. Handles 600° no prob. Even use it on the coal stove in the house. Amazing stuff. Withstands LOTS of heat, & every chemical I could throw at it.

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I use Mobil 1 full synthetic gear lube. It meets the GL5 spec so no additive is needed. I believe this will be fine for your set up as well.

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No additives needed.

 

Use a quality synthetic 75w-90 gear oil and replace the gasket, with a gasket.

 

You can use rtv, if you'd like, but I personally can't stand the mess it makes. If it has a gasket available, use it and install it dry. If there is not a gasket for what you are trying to seal up, that is when RTV comes into play. Sorry, just my opinion on the subject.

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Absolutely correct...

 

What GM says...

 

***Important: Installing any friction modifier causes the clutch pack in the locking differential to slip and miss engagement. A fluid flush can often remove debris that is a contributor to clutch chatter. Adding friction modifier additive can cause a loss of locking differential functionality***

 

What the G80 manufacturer says...

 

"From Mr. Ralph Holmquist of Eaton, the maker of the locking differential:

"The maintenance schedule for the rear axle was developed by American Axle &

Manufacturing and GM truck based on multiple tests. The Eaton locker does

not require additional maintenance nor does it add heat to the lube. The

lube will darken due to the carbon wear on the clutch surfaces, much the

same as a disc brake pad & rotor. This does not damage axle components such

as seals or bearings. However, a new axle can produce excessive

temperatures (plus 350 degrees F) due to the ring & pinion breaking in that

will break the lube additives down. Avoid high loads, trailer towing and

high speed extended driving during the initial break in of the vehicle.

After the break in period axle temps will level at a much lower figure.

Lube changes are a good idea because the additives are replenished and

contaminates such as casting sand are eliminated. The axle is filled at the

factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276.

The GM service # is 12378261. This is the only lube we have done extensive

testing with to insure locker compatibility. The only negative to using one

of these other lubes is an increased potential for clutch chatter. This

really doesn't hurt anything and can be corrected by changing the lube. I

noticed in the latest GM owners manual the term "or equivalent" when

referring to the lube specification. Look for a GL5 rating on the bottle to

make sure the ring & pinion, seals & bearings are protected. Limited slip

additive is not needed."The following information outlines the type of

rear axle lubrication utilized in GM Truck axles.

I. Fluid Type

II. The 1500, 2500 and 3500 GMT 800 Trucks utilize SAE 75W-90 Synthetic Axle Lubricant. The GM part number is 12378261 and the specification is 9986115.

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from a GM service bulletin

 

***Important: Installing any friction modifier causes the clutch pack in the locking differential to slip and miss engagement. A fluid flush can often remove debris that is a contributor to clutch chatter. Adding friction modifier additive can cause a loss of locking differential functionality***

 

video with info

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Most 75/90 synthetics at autozone/advance etc have the modifier already in the lube.

 

Mark

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Thanks for all the info!

 

I found Autozone carries the gasket I will need so that is great. I always prefer to use a gasket when I can as well.

 

About how many quarts of fluid will I need?

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Thanks for all the info!

 

I found Autozone carries the gasket I will need so that is great. I always prefer to use a gasket when I can as well.

 

About how many quarts of fluid will I need?

Approximately, 2.15 quarts.

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Interesting video posted next to the one above on youtube

 

 

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Interesting video posted next to the one above on youtube

 

 

 

There are many Internet experts out there with many opinions. Opinions do vary. When used as the G80 locker was designed and intended, its works great. As with anything it will fail under certain abuses that it was not designed to do. If you do severe off road with big wheels or race the after market is the way to go. For stock trucks as a traction device at low speed the G80 locker works well.

 

It does lock the rear wheels together and does not function like a limited slip that functions all the time and is made to slip around curves.

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GM and Amsoil, do not.

 

Redline has one also. I don't think there is any aftermarket oil that truly meets the GM spec on this.

 

Mark

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Interesting info in this thread. I have a G80 locker and I replaced the rear axle seals last summer and refilled with Royal Purple 75w-90. Not too long after I started noticing an annoying hum from the rear at speed and also more random "clunky" type sounds at low speeds when first hitting the gas off idle, etc. Wonder if I need to get the GM fluid in there or if it's just my imagination. The GM fluid is stupid expensive, though.

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I've got the G80 in my '07 NBS Silverado. Running Amsoil 75W-110 front & rear. Quiet as a mouse.

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