Reflections: Rear differential fluid (8" axle in a V8?)

strat81

Original poster
Member
Dec 29, 2011
399
I had some time to change the rear differential fluid in my 2004 Rainier V8 AWD (G80, 3.73 gears) this weekend. It has ~73,500 miles on it. I purchased it used with 50k on the odometer and was told that the fluid was changed at that time as the selling dealer fixed a rear pinion seal. If the fluid was changed, I don't trust a used car dealer to put in good stuff, so I decided to do it myself.

1) I used just around three quarts of Valvoline SynPower (full synthetic) 75W-90 gear lube. On my test drive afterwards, I noticed no noise or chatter from my G80 axle. I expect this fluid to last 75k-100k miles.

2) Could they have put the fill bolt in a worse spot? The housing interferes with wrenches, breaker bars, etc. It's a 1/2" NPT, if you decide to change it.

3) Nine out of ten cover bolts came out easily with my cordless impact. That last one was blocked by whatever that rectangular bar is that goes across the truck (it's not the sway bar).

4) Filling was easy. I attached 12" of vinyl tubing to the end of the bottle and put the other end in the fill hole. There was enough room under there for the bottle.

5) Remove the spare tire. This gives you tons more space in which to work, but it also gives you a chance to inspect your spare. My spare tire was down to about 25psi of air and it was also mounted incorrectly. I've never had it down so this was not a pleasant surprise. According to the owner's manual, mounting the spare tire incorrectly (valve stem down) means the secondary latch system won't work, so if the winch cable breaks, you lose your spare and create a road hazard.

6) Apparently I don't have the 8.625" axle. I thought all V8s did, regardless of wheelbase. I bought a new gasket (Fel-Pro 8.625" gasket at NAPA) but when I compared it to the old one, it was different. The difference is slight, but enough that it will not work. The gasket that was on there, that I reused since it was in very good condition, looked like this (Fel-Pro 8.0" gasket at NAPA).

Does this mean the conventional wisdom that all V8s have the 8.625" axle is wrong? Did I just not do my homework? Is it possible that the original 8.625" axle was swapped out with this 8" one? Any concerns about towing, hauling, or driving with the 8" versus the 8.625"?
 

The_Roadie

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Nov 19, 2011
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Portland, OR
strat81 said:
...That last one was blocked by whatever that rectangular bar is that goes across the truck (it's not the sway bar).
The technical name is the Panhard Bar.
Does this mean the conventional wisdom that all V8s have the 8.625" axle is wrong? Did I just not do my homework? Is it possible that the original 8.625" axle was swapped out with this 8" one? Any concerns about towing, hauling, or driving with the 8" versus the 8.625"?
Very odd. Maybe the Buick is different? Maybe they assume you will not be aggressively offroading since you don't have the transfer case with the low range? Maybe it was swapped? Maybe NAPA is wrong about that gasket?

When AlekG was researching diff covers and convinced Purple Cranium to offer a diff protector for our unique rear axle, he found the bolt pattern was identical, and only the depth of the covers was different between the two ring gear sizes. So PC only sells one kind of protector. That implies one kind of gasket, but I never looked up the part numbers to confirm that.
 

strat81

Original poster
Member
Dec 29, 2011
399
the roadie said:
The technical name is the Panhard Bar.
Very odd. Maybe the Buick is different? Maybe they assume you will not be aggressively offroading since you don't have the transfer case with the low range? Maybe it was swapped? Maybe NAPA is wrong about that gasket?

When AlekG was researching diff covers and convinced Purple Cranium to offer a diff protector for our unique rear axle, he found the bolt pattern was identical, and only the depth of the covers was different between the two ring gear sizes. So PC only sells one kind of protector. That implies one kind of gasket, but I never looked up the part numbers to confirm that.

Advance lists two different gaskets too (both Fel-Pro), and both say they fit my V8 Rainier:

Buy Felpro Differential Cover Gasket RDS 55028-1 at Advance Auto Parts
Buy Felpro Differential Gasket Set RDS 55479 at Advance Auto Parts

The blue one did not fit, although it was close.

Rock Auto lists the same two gaskets for the V8 and I6 too.
 

strat81

Original poster
Member
Dec 29, 2011
399
I do know the vehicle sustained damaged of some kind before I purchased it (despite Carfax saying it was never in an accident). I do not know the extent of the damage, just that it resulted in a new driver's side mirror, and new paint on the front and rear bumpers, front fender, rear quarter panel, and doors, all on the driver's side. That could have been the result of something minor, such as key incident or trouble getting out of a parking spot. Or, it may have been bad enough to hurt the rear axle, requiring replacement. Or, something else caused damage to the original axle, requiring replacement with a junkyard axle.

Or, maybe axles are like transmission pans and it's a mystery, at least when it comes to 5.3L Buicks.

Nice work on the overlay, BTW. :smile:


ETA - Has the Purple Cranium protector been verified to work with 8" and 8.625" housings? How were the housing sizes verified? For example, if they sent me one last week, I'd have told them, "Sure, I have a V8, so it's a 8.625" housing". But, I'd have been wrong, at least based on all of this from yesterday.

Is it possible GM managed to cram 8" and 8.625" ring gears in the same size housing?
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
strat81 said:
ETA - Has the Purple Cranium protector been verified to work with 8" and 8.625" housings? How were the housing sizes verified?
AlekG invested in TWO covers, one for each size, and sent them to Purple Cranium for them to design their Spider. He said the holes were the same, and only the depth changed, so the hole pattern was the same. I had the first one bought in the USA (AlekG is in Toronto) and I put it on my original 8" rear end. When I destroyed that, and upgraded to the larger diff, the Spider transferred right over. I really, really, can't explain the gasket difference unless FelPro made an error. Would not be the last time an aftermarket parts store data base has an error. (The most notorious of which is the "how many amps is your alternator" question when you go to buy a serpentine belt.)
 

strat81

Original poster
Member
Dec 29, 2011
399
the roadie said:
AlekG invested in TWO covers, one for each size, and sent them to Purple Cranium for them to design their Spider. He said the holes were the same, and only the depth changed, so the hole pattern was the same. I had the first one bought in the USA (AlekG is in Toronto) and I put it on my original 8" rear end. When I destroyed that, and upgraded to the larger diff, the Spider transferred right over. I really, really, can't explain the gasket difference unless FelPro made an error. Would not be the last time an aftermarket parts store data base has an error. (The most notorious of which is the "how many amps is your alternator" question when you go to buy a serpentine belt.)

Could you try your overlay magic again? Pull up a 2009 Trailblazer 4.2L on Rock Auto, and look at those diff cover gaskets. Both are Fel-Pro, but the 8.625" isn't the blue one, it's a black one, but a different shape and part number than for the 8.0".
 

strat81

Original poster
Member
Dec 29, 2011
399
the roadie said:
I had to go to the Federal Mogul site to get THEIR higher resolution images of those two items. Same issue.

View attachment 7460

I think you're right about this being aftermarket weirdness.

I went to Nalley GMC's website and I could only find two part numbers for differential gaskets: 26063649 for SS models, and 12479020 for everything else (SWB, LWB, I6, V8). If that is the case, it confirms your statement above about GM using the same housing for I6 and V8 (non SS) models.

Rear Differential Cover Gasket (2005-2009 9-7X), Saab USA Parts.com shows that it works for "9-7x All models".

So, maybe I do have an 8.625" axle after all.
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
strat81 said:
...it confirms your statement above about GM using the same housing for I6 and V8 (non SS) models....
To clarify for the archives, I only said the BOLT PATTERNS were the same. The housing is indeed different, as everybody observes when they do the axle upgrade. The larger ring gear gets a beefier housing, different axles, internal bearings, etc.
 

strat81

Original poster
Member
Dec 29, 2011
399
the roadie said:
To clarify for the archives, I only said the BOLT PATTERNS were the same. The housing is indeed different, as everybody observes when they do the axle upgrade. The larger ring gear gets a beefier housing, different axles, internal bearings, etc.

What are some other ways to check what I have back there, short of comparing two side by side? Any tell-tale indicators?
 

Sparky

Member
Dec 4, 2011
12,847
The 8.6" housing is deeper (little longer "snout" on the pinion end) and I seem to remember something else being a little different (an extra gusset perhaps). The diff cover was the most obvious when looking at them next to each other. The 8" had a fairly small "hump" in it and it was rounded the whole way. the 8.6" cover hump was much taller and started off almost straight off the flat surface before curving. If it wasn't wet and raining outside I'd go take a quick measurement.

I can confirm the covers otherwise look the same, and therefore also the gaskets are the same between the 8" and 8.6" rear axles.

I think the aftermarket gasket strangeness is because the rear axle housings for other vehicles with the 8.6" rear gears is different (though the carrier is the same). The GMT360 has a unique housing (for some stupid reason). I'll bet that the other gasket that doesn't fit our 8.6" rear axles would fit on a Silverado or something.
 

C-ya

Member
Aug 24, 2012
1,098
To add my story to this thread, I bought the FelPro RDS55028-1 gasket for my '05 V8 EXT since it was only $4.99. I could get at most two holes to line up, and of course the other ones were nowhere near where they needed to be. I put it back together with RTV. It looked like the rear diff cover had never been off, but I do live where corrosives are used liberally about half of the year.

Interesting to see if others have noticed this, but my diff cover had 8.6 stamped in it on the right side, going vertically from bottom to top. I'll try to grab a pic later. Is this significant?
 

djthumper

Administrator
Nov 20, 2011
14,950
North Las Vegas
C-ya said:
To add my story to this thread, I bought the FelPro RDS55028-1 gasket for my '05 V8 EXT since it was only $4.99. I could get at most two holes to line up, and of course the other ones were nowhere near where they needed to be. I put it back together with RTV. It looked like the rear diff cover had never been off, but I do live where corrosives are used liberally about half of the year.

Interesting to see if others have noticed this, but my diff cover had 8.6 stamped in it on the right side, going vertically from bottom to top. I'll try to grab a pic later. Is this significant?
XL's have the 8.6 rearend.
 

C-ya

Member
Aug 24, 2012
1,098
djthumper said:
XL's have the 8.6 rearend.

I know that, and you know that, but apparently FelPro doesn't know that. Still trying to figure out how to get the correct rear diff cover gasket. Dealer, maybe?

I wasn't sure if the 8" rear ends had an 8 or 8.0 stamped/engraved in their diff cover.
 

Sparky

Member
Dec 4, 2011
12,847
Get the gasket for the 8" - same gasket.

Although the original gasket should have been a reusable rubber one.
 

C-ya

Member
Aug 24, 2012
1,098
It looked to be of the reusable type, but since it was on since day one (I think), I decided against reusing it.
 

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