Front differential rebuild GM 8.25 IFS (needs to be a sticky)

[Matthew Jeschke]

 

[swathdiver]

However, I only have one shim on there right now. Where do you order more from?

I would go to the dealer again. You can get them in varying thicknesses under the same part number. Ordering from Amazon you do not know what thickness will come, same for ebay.

 

[Matthew Jeschke]

I notice from the pictures you measured backlash on the pinion yoke. I'm thinking that will be quite different from ring gear backlash, although related. Do you have a spec for measuring backlash on the pinion yoke?

Would be very helpful as many people could diagnose further before opening the case

 

[CountryBoy19]

I would go to the dealer again. You can get them in varying thicknesses under the same part number. Ordering from Amazon you do not know what thickness will come, same for ebay.

I would suspect that you can get aftermarket shim kits for the front, just as the rear.

If so, IMHO, you want Super shims, either from Yukon Gear or Motiv. Both make a shim kit (conveniently both called super shims) that consists of a thick inner shell and a thick outer shell. You sandwich the thin "adjustment" shims between the 2 shells. The BIG difference is on these 2 particular shim kits the inner shell has a lip on in the ID. This lip holds all of the shims plus the outer shell in alignment so they don't slide apart and get damaged during installation. Having just gone through a rear diff rebuild, the Super Shims made a HUGE world of difference.

And yes, I plan to make a thread. I'm not big on pictures, but there are plenty of youtube vids on rebuilding 8.5/8.6 rear ends. I plan to make a thread of lessons learned and advice as well as try to put a lot of the "general info" that isn't necessarily easily found in that thread...

 

[Matthew Jeschke]

Thanks for the shim tip CountryBoy19 I'll have to take that into account when I rebuild the rear (possibly helps on front too). This question is on the front 8.25 IFS diff...

For this measurement I locked truck into 4wd so the axles would keep diff from spinning while I tested it.

So I measured backlash as observed from the yoke (although spec is 0.060 - 0.010" at ring gear). I got 0.085" on the pinion yoke. Not sure if that is equivalent to measuring at ring gear? I don't think it is despite what people are saying in forums, the yoke is a lever connected to the gear. Although possibly yoke is at same height as ring gear or very close?

Then I applied a little extra force and got extra slop out of the diff. This has me a bit worried... I'm not sure what this slop is. Any ideas?

I may need to remove the inspection cover to take a closer look.

 

[CountryBoy19]

Thanks for the shim tip CountryBoy19 I'll have to take that into account when I rebuild the rear (possibly helps on front too). This question is on the front 8.25 IFS diff...

For this measurement I locked truck into 4wd so the axles would keep diff from spinning while I tested it.

So I measured backlash as observed from the yoke (although spec is 0.060 - 0.010" at ring gear). I got 0.085" on the pinion yoke. Not sure if that is equivalent to measuring at ring gear? I don't think it is despite what people are saying in forums, the yoke is a lever connected to the gear. Although possibly yoke is at same height as ring gear or very close?

Then I applied a little extra force and got extra slop out of the diff. This has me a bit worried... I'm not sure what this slop is. Any ideas?

I may need to remove the inspection cover to take a closer look.

I knew you were talking about the front but I had a brain fart when thinking about the shims; they're installed totally different so the super shims don't really get you anything on the front. That being said, Ibet the Yukon/Motiv shim kits are just as good as the dealer offerings.

Lastly, I don't think it's possible to measure gear backlash from the yoke like that. When measuring gear backlash you normally fix the pinion/yoke and measure the ring gear. This ensures the ONLY backlash/slop you're measure is that between the 2 mating gears. If you put the axle in 4WD to lock the axle shafts and rotate the yoke you're measuring the backlash in the entire system, unless the carrier is somehow held stationary in there. That is: the gear backlash + the carrier to pin backlash + pin to spider gear backlash + spider gear to carrier gear backlash + carrier gear to spline backlash + and backlash in the CV joints & hubs... I'm surprised you have such little backlash at the yoke.

In my case, I had bad spider gears and generally just everything in the carrier. Even after setting my gear backlash to .005" (.003-.006" is spec for rear) I still had .25"+ backlash at the yoke because of the loose carrier. That is what was causing my rear-end "clunk".

FWIW, long story short, I have to tear back into it for a minor issue. I was on the fence earlier about a Truetrac but because I still have significant slop because of the worn carrier I'm just going to replace with a Truetrac so it's all coming back apart to fix the minor issue and drop the Truetrac in.

 

[Matthew Jeschke]

Is TrueTrac the posi locker? G80 setup?

I hit my head after I posted the video... I think that big slop is coming from my CV Axles. The upper joints are shot, they have LOTS Of play in them. Guy I bought truck from had put leveling keys in, but not thought about anything else. It ruined a bunch of stuff up front. I lowered the truck back down... and had always suspected the CV(s) but now I know they are loose as I have replaced all the other broken parts (shocks, axle bearings and seals, and sway bar links).

Everybody seems to be measuring the front IFS at the yoke. Makes no since to me other than you could tell if there is a little or lot of backlash. The yoke creates a leaver arm / multiplies the measurement so no idea telling how much more you're reading than at the actual gear. Long story short, I think my internals are good up front.

I blew up my rear end though so your comments will come in use My G80 is sitting on the ground in garage, I threw in a whole new (junkyard new that is) open diff rear end in the back. I want to put the G80 back in there, but a not sure how much metal filings are in it from the exploded rear end :|

 

[CountryBoy19]

Is TrueTrac the posi locker? G80 setup?

I hit my head after I posted the video... I think that big slop is coming from my CV Axles. The upper joints are shot, they have LOTS Of play in them. Guy I bought truck from had put leveling keys in, but not thought about anything else. It ruined a bunch of stuff up front. I lowered the truck back down... and had always suspected the CV(s) but now I know they are loose as I have replaced all the other broken parts (shocks, axle bearings and seals, and sway bar links).

Everybody seems to be measuring the front IFS at the yoke. Makes no since to me other than you could tell if there is a little or lot of backlash. The yoke creates a leaver arm / multiplies the measurement so no idea telling how much more you're reading than at the actual gear. Long story short, I think my internals are good up front.

I blew up my rear end though so your comments will come in use My G80 is sitting on the ground in garage, I threw in a whole new (junkyard new that is) open diff rear end in the back. I want to put the G80 back in there, but a not sure how much metal filings are in it from the exploded rear end :|

The Truetrac is a maintenance free limited slip. But unlike a normal clutch type limited slip that transfers less torque the more it slips (due to heat build-up in the clutches etc), the Truetrac's unique, patented mechanism bites harder (locking the 2 wheels together) with more torque and slip. I was skeptical of the mechanism so I looked up a few youtube videos. You can jack 1 wheel up off the ground, block the fronts so the truck won't move, and the single drive-wheel touching the ground will drive pretty hard at nearly the same speed of the free-spinning wheel. It will squeal the tire on the pavement. Everything I've seen and read on it has reinforced that it's a truly amazing differential. One that performs as good as the factory G80 without the delicate engagement mechanism of the G80. The difference between a clutch-type limited slip and the Truetrac is that any time the wheels are spinning at a different speed there is clutch-slip & resistance with the clutch-type. This means they have to balance the resistance of the clutches with driveability. The Truetrac doesn't really start biting until there is an actual torque difference, so during street driving, the speed difference around a turn doesn't cause much bite because there isn't really a difference in torque, even though there is a speed difference. It's not until 1 wheel begins losing traction, causing a torque difference that the helical gears start to jam up in the housing and send torque back to that wheel.

As to your axle, if you blew up your G80 there are bound to be metal pieces in your rear axle assembly that are eating all your bearings. I would rebuild that as soon as you can. Clean the housing good. Many say to take it all the way out and pressure wash it. I used lots of brake cleaner, rags shoved through the tubes with a long rod, and LOTS of cotton swabs to clean out the recess behind the carrier bearings.
FWIW, the cheapest place I could get the Truetrac was Summit racing. $469, and right now they're running $50 off $500 order and they ship it for free if the order is over $99. Because I have to pull it apart a 2nd time I decided I'm not doing that again so I'm putting all new parts in the whole rear axle minus the ring & pinion which look perfect. I tacked a couple bearings or something onto the Truetrac to get that order to $501 and then placed a 2nd order with the new axles and the rest of the parts for $257 (minus $25 on that order). Considering the rebuild kit for the factory G80 is ~$200, you have to actually do the rebuild work, and you still have a weak "bomb" it's worth the $420 to have what many call the best street & multi-use differential you can have.

Oh, one last thing. If you do buy a Truetrac you cannot use the standard carrier bearings. It takes a different bearing and race. LM603012 & LM603049 as called out in the Eaton application guide.