Rebuilding the front diff -- tips and tricks

[Reprise]

Rebuilding (sorta) the front differential – 7.25 IFS (IOP)


I haven't seen much online concerning this diff, so thought I'd put some info on it here, since we're the main users of it. I think a version of it was also used in the S10 / S15 pickups, but the case and output shaft on that one are different than ours. Some references to it will include 'IOP', which is 'In Oil Pan' (that's us).

My adventure started with the realization that the fluid had leaked out, which is actually pretty common with these. These appear to come from the factory with anaerobic sealer joining the case halves, in lieu of an actual gasket. Obviously, that solution doesn't work, so we'll find something else.

The second cause of leakage is from the outer seals. Given that the diff had to come out to reseal the case (and that my Envoy is now 20 model years old), I decided that I'd replace the seals as well, and give the internals a look-see (and I'm glad I did).

If you're pulling this out of your truck... well, get ready for a lot of work. You'll be pulling both CV axle shafts, the splined disconnect (which often is stuck to the oil pan), the intermediate shaft (in the oil pan), unbolting the diff (but not removing it), then the oil pan itself (especially if you have the V8, like I do). After all of that, the diff can be removed.

My disconnect actually came off the pan easily – it wasn't stuck at all. The differential was a different story. What worked for me was a set of pickle forks (ball joint separators), along with a few hours spent wedging them into different areas between the case and the pan. I do not recommend hammering on the ends / edges of the case, trying to break the bond between the case and the pan (like you can for the disconnect); I wound up cleaving off a 3” x 0.25 wide section of the case, doing that. Thankfully, it was on the edge, didn't affect the fluid holding capability, and I was able to use some JB Weld to reattach it.

So, we'll assume you have the diff out of the truck, and are ready to start working on it.
Before turning a bolt on the case, I highly recommend watching the following South Main Auto tutorial, to familiarize yourself with the workings of GM's front differentials used in trucks (this one is from a Tahoe, but it uses the same type of side adjusters that ours does). It's about an hour long, but it's time well spent if you're new to this. I watched it a couple of times, referring back to it when needed, as I've never set up gears before, and this diff uses adjusters vs. shims (like on rear diffs).

SMA -- rebuilding front diff / gears (Tahoe)


I'm going to split this into three parts – 'seals only', 'seals plus case', and 'seals / case / bearings'

I wound up doing all of these (except the pinion bearing, which I judged to be good). If your pinion bearing or pinion oil seal needs replacing, this write-up is not going to fully help you, and unless you have experience setting up a differential, I'd recommend having a professional set up yours, once you've torn it down. I spent a fair bit of time on adjustments, and I didn't even touch the pinion bearing.

'Seals only' (do this if you're sure your leak is only coming from here, and not from the case seam)

You'll need:

- National 710687 and 710688 seals (left / right sides, respectively)

- A seal puller (the seals are thick and rigid, and can be difficult to get out. They also function to lock the side adjusters in place).

Whatever method you prefer to drive the seal in (a seal driver kit is nice, but many people use blocks of wood, etc.)

First, match up your seals to each side of the ones installed on the diff. You'll see the one on the driver's side has a wider opening than the passenger side (which you normally can't see, as it's on the oil pan side). That's because the driver's side has the CV axle attaching to the diff, while the passenger side has the intermediate shaft attaching to it.

Once you get the seal off, you're going to see the side adjuster underneath it. DO NOT MOVE THESE. They shouldn't really move, with the tension of the side bearings / carrier in back of it, but consider yourself warned. I'd also replace one seal at a time, in order to ensure the adjusters stay in the position they're at. If you clean the top of the adjuster, you'll see that one of the 'tabs' has a very small diamond-shaped dimple in the center (top center in photo below). This is your reference point. If the 'wheel' of the adjuster spins freely, make sure it's in the same place when you drive the new seal in. Clean gently.


(One of the side adjusters removed. Note the dimple at the top; it'll be hard to see unless you clean the surface. Also, the press-in inner needle bearing is shown here – mine were both good)

If you turn the old seal over, you'll see a bunch of small tabs surrounding the outer edge / lip. And you'll probably see that all except two are pushed against the back of the seal. The two that still hang down are the ones that kept the side adjuster in place. (note: the SMA / Tahoe demo locks the adjuster via a separate metal tab that inserts between the adjuster and the case – that's where these two differ).

GM says you don't have to line up the tabs on the seal with the slots on the adjuster. But I'd try to make sure at least two were lined up, on opposite sides. There are 12 tabs on the adjuster, and 14 on the seal, so I guess they figure that's how two of them will not compress, and hold the adjuster in place.

Clean the housing, optionally smear a little lube on the inside edge of the case housing, and drive the seal in. Done. Up to you if you want to run some sealant on the outer edge of the case halves as 'insurance', but if it does develop a leak, don't expect that to hold too long. I would also clean, deburr, etc., the 'lip' of the passenger side of the diff that attaches to the pan, and coat it with anti-seize, in case you ever have to repeat this exercise. And, given the effort needed to get the diff out of the truck, and depending on the age / mileage... why not ensure the internals / case seal is good, as well? But if you're sure the seals were your only issue, and you're a person of faith... then go ahead and reinstall the diff.


'Seals plus case'... For this, you need all of the above parts from the last section, along with a tube of sealant. I used Permatex 'Ultra Black'; it was the best one I found at the local parts store.

Also needed: 15mm wrench / socket, torque wrench.

Pry out the oil seals as per above, keeping track of the adjuster positions. (I wouldn't put the new seals in just yet, as you may decide to 'go further', once you see the internals.) Then split the case. The ten (10) 15mm bolts that connect the housing together are all the same length, so no need to keep track of which one came from where. Put them aside for now.

Set the case so that the larger half (driver's side) is down, and the smaller half is facing up. To split the case, I didn't really have to pry much. There's a lip on the one end ('front', as it's installed on the truck) that will allow you to get a large screwdriver or pry bar in to split the case, if needed.

With the case halves separated, you'll notice that it didn't appear to have much in the way of sealant connecting the two halves. Time to clean. I used a sanding drum on a rotary tool to prep the surfaces.

From here, it's not a problem to check the bearings. You'll be looking at the inner / passenger side. Check both the outer race (which is in the case half) and the bearing (pressed onto the carrier shaft). My outer races on both sides were pitted, so I decided to replace the bearings. More on that later. You can lift the ring gear right out of the bottom assembly and check the bearings on that end, too. Note that the outer race on this end may be very loose in the housing, and, with no tension, the side adjusters will both now move freely, if you're not careful (see: 'The ring gear...' in the next section, below).

With the carrier out, you can now inspect the following:
Carrier gears (this is an open diff, so there's no locker)
Ring gear
Pinion gear
Backlash / any noise on pinion. Basically, you want to make sure that when you spin the pinion gear, that you don't have any noise, and that the 'free movement' until the yoke starts turning the pinion is small. Also, look at the area behind the 'dust cover' (black plastic, just behind the yoke) to make sure you don't see an obvious oil leak. If you do... then it's advisable to replace the oil seal (but you have to disconnect the yoke and pinion nut to get to this, which I don't go into here, and this is where things get tricky, if you don't have experience with this).

If you're sure your bearings / gears are good, apply your sealant and reassemble the case halves per the directions on the sealant. For the Ultra Black, this means that you cover one side of the housing mount surface, including around the bolt holes, using enough so that it will 'squeeze' out of the side of the case (but it won't squeeze out until it's torqued). I wound up using approximately 1/3 of the tube. Then, place the two halves together, but only snug the bolts in (or, 'finger tight'). After one hour, then torque the bolts to spec (which is 35 ft-lb). Wait 24hrs before filling with fluid. Reassemble diff and other parts, and you're done. But if you need to replace bearings, continue on...
<End of part 1>

 

[Reprise]

<Part 2>
'Seals / Case / Bearings'...

For the bearings, you're going to need...
- The bearings (carrier: 'A18'; adjusters: 'HK4012') – bearings in good condition can be reused!

- Gear marking compound (yellow will provide more contrast; I could only find Prussian Blue, locally. It worked, but I'd prefer yellow, if I could've gotten it that day).

-Most likely, a bearing separator. I got mine at HF as a kit for $42, with 15% discount; Amzn is similar.

The ring gear will lift right out of the housing. A couple things of note:

The outer bearing race on this side is a very loose fit, and is basically held in place by both the side adjuster and the carrier bearing race. If you turn this case side over, don't be surprised if the outer race actually falls out of the case(!) It did with mine. Also, as an FYI, there is a compound available to hold bearings / races in place, but I would not use it here, as I believe the looseness is by design (more on this later; recall what Eric explained in the video, which is that the majority of the side-to-side adjustments are done on this end, vs. the passenger side end.

With the carrier removed from the case halves, the adjusters will now spin freely if moved. Be aware of this, and use it to your advantage. Note the reference point (dimple). Mark the corresponding area on the case half, if needed. Take your thumb and place it on the outer side, and as many fingers as you can fit in the opening to grab firmly onto the adjuster, and count exactly how many counter-clockwise turns are required to remove it from the housing (they're fine threaded). Record this, as it will likely be different for each side (mine were '8' on the passenger side, and '6' on the driver's side – but I didn't get to count the 'fractional' turns past there, which caused me to have to make more adjustments, later). Try to get it exact as possible, as the more precise you are when reassembling, the fewer side adjustments you'll (hopefully) need to make when you put things back together. Also note that the adjusters are different on each side, and they're not interchangeable, so don't mix them up!

By the way... there is a needle bearing in the inner surface of the adjusters... HK4012 (same both sides). Mine were just fine, so I left them as-is, but if yours are bad, or you just want to put all new bearings in, now would be the time to do so. If you notice that the bearings are worn, now is also the time to carefully inspect the splines on your CV axle and intermediate shafts, and replace as needed.
You can replace the adjusters as a unit (they come with new bearings), but Rock is charging about $150 / each for them, and it's pretty much a 'GM only' part. So it's much cheaper to press in a new bearing.

On to the carrier bearings...

They're the same for each side. You may note that if both are bad, that the 'cup' is bad on one side, and the 'cone' is bad on the opposite side. With mine, I had pitting on the cup / outer race on both, although one side was way worse (and I think I caused some of it, while I was trying to pound out the diff from the passenger end – again, don't try this, it doesn't work. See pic below).


(Pitting shown – right side carrier outer race. This was by far the worse side, of the two)

The National # / cross-reference for the carrier bearing set is 'A18'; they're commonly available, and fairly cheap. Also, if either the 'cup' (outer race) or the 'cone' (tapered bearing) is bad, the bearing should be replaced as a set. For the cup, this isn't hard at all. It'll tap right out on the lip from the outside end, without too much trouble (on the driver's side, it'll probably fall out, as I mentioned above.) On the passenger side, it'll drive in with gentle tapping (you may even be able to press it in by hand, but I did use a driver). Expect about 1/16” free space at the bottom the housing, once the cup / outer race is seated.

The bearing race / cone is the harder end. There's very little room between the bottom end of the bearing and the carrier (the bearing sits nearly flush against it), and I decided to just swallow the $40 and get the proper tool (you may be able to find it on rental, too.) AAM left you a small slot on two sides, if you want to use a jaw puller to get it off, but mine weren't close to fitting them.

When using the separator... the bearing will pull off better if you tighten each side of the separator in the housing a bit, before attaching the puller / jackscrew. Also... grease the jackscrew, as you would with any puller. For this bearing, you won't be able to get much of the separator on the bearing, so use a wrench / socket to tighten each half down; otherwise, it will likely slip off.

You'll also need to put something between the jackscrew and the shaft, narrower than the inner edge of the bearing race, but that will fit around the shaft. I used an appropriately sized impact socket (I think it was a 17mm), along with a bolt placed into the square (ratchet) end of the socket (the bolt becomes sacrificial). You can also use washers, etc. The goal of this is to keep the jackscrew from bottoming out while you're pulling.

I found after a few turns by hand, that my cordless impact was the right tool for turning the jackscrew (another 17mm). Use short bursts; it'll pull the bearing free after just a few shots. You use short bursts to ensure everything is lined up correctly, and you're not damaging anything (most of all, the shaft), or fighting against yourself to get the bearing off, because the socket is sitting on it.

Clean / examine the shaft on the carrier for any grooving, etc. You can drive the race onto the shaft without too much trouble (I used a block of wood, and no lubricant). When fully seated, it'll be flush both on the top end with the shaft end, and on the bottom with the carrier housing.

With the carrier bearing(s) replaced (if one end has a good bearing on both sides, it can be reused), it's time to reassemble the carrier in the case and set the adjusters / carrier bearing preload / backlash.

Here are the settings, from the factory repair manual:


Here, I'm going to point out another difference between the SMA video and our adjusters.

On the SMA video, you'll see / hear Eric saying '55 ft-lb' of preload on the side bearings. But remember, his aren't threaded, like ours are, and if you try and put 55 ft-lb of torque on our threaded adjusters... well, it ain't gonna happen. So the '55 in-lb', as referenced in the manual, s/b correct.

Now, there's two ways to move the adjusters. One is with two (!) special Kent-Moore tools, J-42213 and J-45224. I actually found J-45224 (right side adjuster) on eBay for $12 shipped, and bought it... but the other one (left side) was over $100 (used). In a perfect world, I'd have both, as it saves time. But I wasn't paying + $100 for a tool I'd likely use... once.

If you don't have the tools, then you can move the adjusters by hand, but you can't have any weight on them when you move them (so the carrier has to be off the housing, and you'll likely be bolting / unbolting at least 4 bolts on the case, several times (it's not too bad, with a cordless impact).

If you had the foresight to record exactly how many turns each adjuster was screwed inward, you'll have a very good starting point. Note that the thread will 'catch' at 1/4” turn – once it does, start counting from there.

Next, get your marking compound, and put a good coating on at least 5 (consecutive) ring gear teeth, on both sides (which are called 'drive' (the convex side) and 'coast' (the concave side)). The oil fill hole can be removed for access, if you want to do this and view the result with a closed case – but I just took the carrier out both when I needed to apply the compound and view the contact patch afterward. You don't have to torque the case down for this – just snug the bolts. I also only inserted the four in the middle closest to the carrier, not all 10.

With the compound applied, rotate the gear a minimum of three (3) turns in each direction (forward / reverse). Then examine the teeth on the ring gear where you applied the compound. You want the exposed area to be as close to the center, on all sides, on both the drive / coast side. You'll note that if it's off from side to side, that it'll be at opposite ends (which are called the 'heel' at the outer end, and 'toe' at the inner end). This is where you'll move the adjusters to fine-tune exactly where the pinion gear contacts the ring gear, side-to-side. If it's significantly 'off', you'll introduce wear on the gears and eventually have to replace them – so try and get the contact point as centered as you can on both sides.

Per the service manual (and SMA video), each 'slot' on the adjuster wheel will move the carrier / ring gear .003 (three thousanths of an inch), which ain't a lot. But make small adjustments at first, until you get the hang of it. Also – the adjusters are to be moved in opposite directions – so, if you move one side inward, you're supposed to move the other side outward, ostensibly by the same amount (but not always).

After a fair bit of playing, this was as close as I could get to 'perfect'. G*d knows I tried my best...




Once you've got the contact area set, then you can seal up the case, as in the prior section (the adjusters shouldn't move too freely), and then put in the new seals (which will also lock down the adjusters). At that point, you're pretty much finished. Remount the diff, and add fluid (75w-90) once the case sealer you chose has fully cured. The fill and drain plugs each torque to 24 ft-lb, and the four (4) mounting bolts (that connect it to the oil pan) torque to 63 ft-lb.

When reassembling, you may want to pre-position the front driveshaft, if you removed it outright, prior to remounting the diff.

Parts cost (two seals / two bearings): Roughly $100 USD retail.
I paid a little more for National, but they offer a 3-year warranty, which is longer than almost everyone else (we won't go into who makes GM's seals / bearings... hint – same company)

 

[Mooseman]

Nice write up Mark. Added to the FAQ.