Rear Diff Oil Change

stingray · Oct 24, 2022 at 5:04 PM

fritzthecat said: ↑

Ok, so I've seen enough buzz on here about changing the oil in the rear differential that you all have finally shamed me into doing it. I've seen some posts about oil brands, friction modifiers, hints, techniques, etc. but for the life of me, I can't find the one that tied most of that together, so I'll just start one.

Wow. This couldn't have been any easier! If you're a do-it-yourself-er, I recommend having a go of it. Thought I'd share a few words and pictures if any of you are thinking about trying it. There's a fair amount of clearance (Clarence ) under the rear axle, but I jacked up the back and put it on jackstands anyway, 'cuz I'm old and I don't like squeezing under cars any more.

First tip: "Don't open the drain plug until you've opened the fill plug". Good advice. I did the top one first. Mine required a 10mm allen-type wrench, I have some that fit on a 3/8 drive rachet. Worked like a charm, no problem getting either one open. Here's where they are:

Filler Hole (on side of rear diff):
View attachment 8286

Drain Hole (on bottom):
View attachment 8287

The drain plug is magnetic. This allows it to collect any bits of metal that wear off so they don't accelerate the wear. Mine had a little bit of mung, but not bad for 70000 miles.

Drain Plug:
View attachment 8289

Scraped off so you can see how much is there:
View attachment 8290

There was some discussion of how much new oil put in. I think the consensus was that the diff holds 1.3 quarts of fluid. There's also talk of "How do you know you got it all out?" "Can you be sure you're not overfilling it?". I used the measure-what-came-out-and-put-that-much-back-in method. Near as I could tell, about 1 quart and 12 ounces came out. That's pretty close to 1.3 qts, so I there's probably not enough left in there to worry about. I used a measuring thingy in the catch pan to get most of it, let what wouldn't fit drain into the pan, then poured that in the measuring thing after emptying the first quart.

First quart:
View attachment 8291

Second 12 ozs:
View attachment 8292

Everyone seems to rave about Amsoil, but that's hard to find. Not sure they sell it in stores. I know you can buy it online, but I didn't want to wait. The parts store near me sells Valvoline, so that's what I used. It says on the container that it meets all specifications and includes the necessary friction modifiers for modern-day limited-slip differentials, so seemed like a winner. Valvoline has the flex-pack, too, which worked well to fill it back up. You're supposed to be able to just stick the nozzle in the fill hole and squeeze, there's room under there for the bag and your hands and everything, but it looked a little awkward, so I used a length of plastic tubing on the nozzle to reach the fill hole and then I had plenty of room for my hands to squeeze the pack. You can see the tube in the next picture.

VERY IMPORTANT STEP!!: Remember to put the drain plug back in before refilling the diff! Not that I forgot this time, but it has happened in the past...

'Nother tip: Don't tighten these plugs down like there's no tomorrow. They have a rubber gasket, so they don't need to be that tight. I've found that a 1/4" drive rachet works well, since they are usually kinda short, so it's a lot less likely that you'll reef anything down too much. Ok. Check.

Putting in the first quart was a no-brainer. Squeeze 'till it's empty. Doh! But, how do you measure out the other 12 ozs? I used the weight method. Mail scale. Weigh a full pack. Divide by 32. Times by 12. Subtract that number from the full number. Looks like it got pretty close. Not sure it's all that critical:
View attachment 8293

So, whole job took me about an hour. Including jacking up the car and letting it back down. And I was being careful. Now that I've done it once, I could probably get through it in a half hour. Not a tough job at all.

If you've been thinking about getting this done, and you're hesitant about tackling it yourself, hope this helps you decide!
Click to expand...

Amsoil is easy to get. Also another great Fluid is Redline.

392hemiTA · Oct 25, 2022 at 2:11 PM

Have any one swapped the 308 gear for the 391 that comes with the 6speed manual, back in the day I swapped my 323 for a 391 in my 70 challenger it became a hard pulling automatic, just wondering what the A8 would be like, any ideas

Wm TPA SXT · Oct 25, 2022 at 4:21 PM

Depends on the Rear End and Year as to which one to use mine takes the 75W 140 which was changed this AM.

Moparisto · Jan 20, 2023 at 7:11 PM

2009 Classic B5 said: ↑

When I worked at Chevron, the gear oil group developed Delo Gear Lube with borate additive. This additive works differently from sulfur-phosphorus additives in that it forms a solid film on the gear face instead of just reducing wear of metal to metal contact. The end result was after 2 million miles, a HD Diesel gear set had zero wear on the gear faces and the whole unit was in as new condition. Outstanding performance compared to conventional lubricants.
Delo Gear oil is available at AutoZone and other places, at least in the Denver area. You will need limited slip additive as with all gear oils unless it is included in the formula. It will say on the bottle if LS is included or not.
Click to expand...

Well, now THAT is a new one on me.

Noted for the archives.