Thump thump, G80 and confusing gear oil

[Road Trip]

I can imagine someone throwing a drum onto a wheel-balance machine, finding the out-of-balance amount and location, and then welding a tab to the drum as a correction.

I've never seen this actually done, but I suppose it has been. No reason not to, brand new drums often have weights welded to them for balance. Or--they've ground-away iron to lighten the heavy side. I've seen both ways.

Up 'til recently I've only seen added weights welded on by the factory,
and have seen them more often as not. (In other words, the
majority of the drums as cast need a bit of help to meet the
manufacturing balance spec?)

That is, until I recently opened up the rear drums on my '99 C2500
equipped with the JD7 brakes. Never seen this before...it took me a minute to figure out
what these huge divots were supposed to accomplish:

You must be registered for see images attach

NOTE: On the other drum on the same truck, the 2 divots that were ground were so small as to have barely broken through the outer rim?

And these drums, seemingly at opposing ends of the limits of the
manufacturing balancing process, drove dead smooth at all speeds.

So smooth in terms of cruising on the interstate as well as during braking
I didn't want to tempt fate by replacing them with brand new drums.
(I've read the tales of woe about new drums here in the 21st century
introducing vibration where there was none previously.)

When I tracked down the single remaining shop in the greater Syracuse
area still turning drums, they (of course) checked the inner diameter
before they would machine them. Incredibly they discovered that these
drums had never been turned before, so at my request he turned
them the bare minimum needed to clean them up. Nice work!

Here's a factoid: Using the time-honored method of first weighing
yourself, followed by weighing yourself while holding the part in
question, the drum above checked in at a hefty 64 pounds on
a scale I trust. (ie: agrees with the calibrated one at the VA hospital.)
Shoot, these are more like flywheels disguised as rear brake
drums! :0)

BTW, that's fully assembled, including both bearings & seal.

****

No doubt your drums are a bit different than mine, but
just wanted to illustrate the 2nd balancing method that
Schurkey mentioned.

Best of luck getting your stuff sorted out. Cheers --

 

[firestoness]

If you have up-and-down play in the wheel bearings, you need to start with the bearings

 

[weaponoffreedom]

I think there is some conflicting info on this topic, and have argued several times before, but not on this forum.

The issue seems to be that some think that the G80 locker DOES NOT REQUIRE a friction modifier. There is a bunch of conflicting info on this, most of which are peoples BS opinions and wannabe scientist........with that said.......

Taking these two as an example:

Valvoline Synpower 75w90.....https://www.valvolineglobal.com/en/flexfill-full-synthetic-gear-oil/

Mobil 1 75w90......https://www.mobil.com/en/lubricants/for-personal-vehicles/our-products/products/mobil-1-syn-gear-lube-ls-75w-90

......between the two, are likely the most popular gear oils used by the common folk, widely available at every parts store, and Walmart for that matter........

both of the above oils listed HAVE a a friction modifier..........OK? That is what the "LS" means......it has the proper additives for a limited slip differential............100% period......

So, returns the question, does the G80 require a LS additive?

I will argue from two points:

1.) I have hundreds of thousands of miles on GM trucks, all using Either Mobil 1, Valvoline, or Amsoil gear oil, all of which contains the modifier. I have on many occasions, caused the locker to lock, while driving, or offraoding, or towing while off roading........and have never had a failure at all. Further, I change my own gear oil and observe the oil and pumpkin and clean the magnet, and have never had any wear issues of any kind. One truck, with 80% towing a 15000 lbs trailer, sold with 230000 miles on it, with no issue.

2.) The GM truck is the most widely sold truck in the USA, contrary to what Ford homos want to say in their commercials. GMC and Chevy are the most sold trucks.......period.....So you mean to tell me that the most common truck in the USA, and by proxy, the most common rear end in trucks in the USA, cannot use these two gear oils? Mobil and Valvoline makes these oils so that you CANNOT use them in GM rearends? NO NO NO. Ridiculous.

In closing, I have used a non LS gear oil, and had issues with the locker unlocking (supertech, which clearly states for "top-off" of LS diffs only) changed back to Valvoline, and it worked puuuuuuurrrrrrfect.

There is nothing special about the G80.

 

[454cid]

I think there is some conflicting info on this topic, and have argued several times before, but not on this forum.

The issue seems to be that some think that the G80 locker DOES NOT REQUIRE a friction modifier.

GM issued a TSB on this because it can interfere with the G80 locking, but I'm not going to argue. Run whatever you want. I've got some 80/90 in mine that's older than the truck.... although, I've got an open diff anyway.

 

[Hipster]

There is nothing special about the G80.

So if that's the case why the f*k you asking? You think you're a better engineer than the engineers that designed it? There is a recommended fluid for it. G80 is not a friction type LSD like an Auburn or many Eaton's. It actually is a Gov-Loc lock locking diff.

 

[weaponoffreedom]

M issued a TSB on this because it can interfere with the G80 locking

great, this mean nearly nothing

So if that's the case why the f*k you asking?

I am not asking **** *****. Dont cuss at me for no reason.

 

[Erik the Awful]

I'm no pro at RPO codes, but I believe "G80" can refer to either a clutch style limited-slip or a Gov-Loc. If you have a clutch style, run the additive. If you have a Gov-Loc, don't. The trouble is you have to know which diff you have, and whether or not your fluid has additive. Because life isn't already complex enough.

 

[Schurkey]

GM issued a TSB on this because it can interfere with the G80 locking

Copy 'n' paste of the GM bulletin attached.

The bulletin was apparently written by a middle-school child. It's vague and ambiguous; and of course it's now so old that the "recommended" fluid part number has changed at least once.

The important part of the bulletin says:

The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.

But they fail to address "what is an additive?". MAYBE they intend this to mean "don't put a lil' bottle of friction-modifier into your gear lube, like we would recommend with a traditional "posi"".

MAYBE they mean "don't use a gear lube that already has friction-modifier blended in at the refinery, or at the bottling plant."

Without a bottle of the recommended lube which MIGHT say whether it's got some amount of friction modifier blended-in, there's no way to know whether GM means "NO FRICTION MODIFIER AT ALL", or merely "Don't add MORE friction modifier to the gear lube that already has some minimal amount blended-in at the factory".

The currently-recommended gear lube for G80 Gov-Locks is--apparently--the "Grape Juice" stuff. It's really expensive, and synthetic lube is not compatible with the ORIGINAL silicone sealer used on axles made before a certain date--which is the subject of a different GM Bulletin.

For the record, I filled my new-to-me 9.5" Gov-Lock rear axle with Amsoil gear lube having some friction modifier blended-in at the factory, and my Gov-Lock soon failed to engage. Took two changes of fluid to old-style, non-synthetic, zero friction modifier gear lube to bring it back to life. My suspicion is that the poor thing has worn clutches and the friction modifier pushed it over the edge in terms of functionality. MAYBE if I replaced the clutches, I could use synthetic fluid with a minimal amount of friction modifier already blended-in.

 

[Schurkey]

I believe "G80" can refer to either a clutch style limited-slip or a Gov-Loc.

Yes. Trucks get the Gov-Lock, cars got "limited slip". There are some exceptions.

If you have a clutch style, run the additive. If you have a Gov-Loc, don't. The trouble is you have to know which diff you have, and whether or not your fluid has additive. Because life isn't already complex enough.

What makes it more complex still, is that both Gov-Lock and "limited slip" both use clutches.

 

[weaponoffreedom]

MAYBE they mean "don't use a gear lube that already has friction-modifier blended in at the refinery, or at the bottling plant."

Yes, this IS what they mean. Like Lucas oil stabilizer and such.