GM 8.6” IRS rear differential rebuild for more hp

[1LE Fanatic]

To give some background I bought my car July 5th 2019 and I absolutely love it. Always makes your day when kids run up to the car and can’t stop smiling and looking at it.
Anyways as my garage states my car has some modifications to it and is pumping out 501whp to the tires. Which is great and all but I’ve had a lot of issues with the rear diff being chewed up. I’ve rebuilt the diff once with a JRE rebuild kit and billet aluminum main caps which are said to be the best replacement for the weak cast caps from factory. I meticulously spent a day measuring and painting to make sure I got the diff the best I possibly could and not even 5,000 miles later I’m getting an absurd amount of metal shavings and the tolerances inside the diff have loosened up.
After seeing that but only noticing tolerances loosen .005 - .010 I decided to just readjust everything to be within spec. So it’s probably been less than 1,000 miles since that and I believe I’m getting growling and metal shavings still from the diff.
Are these diffs not capable of holding 501whp and that’s why I’m chewing them up or was it something to do with the way I rebuilt it? I particularly have the 1LE so I do have 3.92 gear ratio in it. Not too sure if that changes much but would like to get some opinions/ideas thrown my way from more experienced diff builders.
Would things like heavier diff fluid be better to run for the higher hp as to “cushion” things more? Also when building diffs and they give a tolerance range to hit should I aim for a tighter tolerances or looser? I’ve tried looking up these questions and can’t seem to find what I am looking for. Anyways I’m open for discussion if anyone wants to spitball ideas with me. Thanks!

[TheFlyingBananaSS]

I have a completely stock rear with Richmond 3.91's. 600rwhp on a mustang dyno. Ran Atco all day, no issues. The rear should hold up to around 800 so you definitely got something going on.

[Machh5]

If the oil is full of shavings its going up beat the internals up quickly. I replaced the carrier in my 4th gen with a new Auburn. After a few 100 miles I checked the oil and it just full of metal to the point it looked like anti seize. I changed the oil and 100 miles later same thing. I pulled the Auburn carrier and put the torsen carrier back in. After no more trouble. Only thing it could have been was the metal clutches in the Auburn.

[Aqua Blue RS/SS]

Something is wrong. They will live just fine at 500 rwhp. Not doubting your rebuild but you might find a reputable gear/drivetrain shop and have them look it over. A second set of eyes and hands on it sometimes can help find things you might have overlooked. Also are you running sticky tires? Do you have rear suspension mods? Wheel hop? There are several things that can contribute to rear failures.

[Greenhornet2]

I am not sure you mechanical background but there is a LOT to rebuilding a diff. It is not something easy and there are more measurements than just backlash. pinion depth is overlooked a lot with novice rebuilders. Proper torque of the pinion nut to get the correct amount of preload is often overlooked.
Using an inch-pound torque wrench and tightening in small increments, measure the rotating torque of the pinion until the reading is the same as the rotational torque noted at removal. If the rotational torque during removal could not be recorded, tighten the pinon nut in small increments until a rotating torque of 2.45–3.70 Y (22–33 lb in) for the 195 mm rear differential or 3.18–4.7 Y (28–42 lb in) for the 218 mm rear differential is achieved.

1. Using a torque wrench, tighten the nut on the J-21777-43 until the specified rotating torque is obtained.
2. For the 195 mm axle, 2.0–2.6 Y (18–23 lb in) for NEW bearings or 1.0–2.3 Y (9–20 lb in) for used bearings.

• For the 218 mm axle, 2.1–3.1 Y (19–27 lb in) for NEW bearings or 1.0–2.3 Y (9–20 lb in) for used bearings.

1. Rotate the assembly several times in both directions in order to seat the pinion bearings and re-check the torque.
2. For the 195 mm axle, if the torque is less than 2.0 Y (18 lb in) for NEW bearings or 1.0 Y (9 lb in) for used bearings, tighten until a rotating torque of 2.0–2.6 Y (18–23 lb in) for NEW bearings or 1.0–2.3 Y (9–20 lb in) for used bearings is obtained.

• For the 218 mm axle, if the torque is less than 2.1 Y (19 lb in) for NEW bearings or 1.0 Y (9 lb in) for used bearings, tighten until a rotating torque of 2.1–3.1 Y (19–27 lb in) for NEW bearings or 1.0–2.3 Y (9–20 lb in) for used bearings is obtained

I don't rebuild diffs I leave it to others as they can be finicky as well. Would also have to see your contact patch to assist with possible issues.

[1LE Fanatic]

As for some more background I am a mechanical engineer and just about everyday I rebuild, fix, or work on something new. I understand torque specs completely and how gears/bearings should work. Rebuilding diffs however is something that is new to me. Hence why I am asking the question. But I can understand everything so keep throwing information at me. The only thing I didn’t replace last time was gears and the locker. I left the locker alone which may be the source of the shavings as it was completely filled last time and the locker is almost completely sealed off besides a few smaller holes.

As for the actual rebuilding of the diff I checked pinion preload with a bend style inch lbs. torque wrench and if I remember correctly got it to 20 inch lbs. (after snapping a snap on breaker bar and absolutely blowing my mind as to how much torque it took to start crushing). FYI I measure the crush sleeve with calipers and pre crushed it in a hydraulic press .012 to relieve some torque needed.

As for the carrier bearings I measured the JRE caps down to .001 of accuracy when getting those to size. I made sure the shims were a tight fit but not overly tight. (I don’t have a case spreader.). I think those got torqued to 48 ft lbs.

Then I measured backlash of the ring gear as well as the pattern and the pattern was almost perfect. Which I never changed the gears themselves so I left the shim on the pinion to set the depth.

The two pictures that I have attached is the pattern about 1,000-2,000 miles after the rebuild. You could not physically feel any slop in the setup but the tolerances had opened up a little.

Keep on adding suggestions! I greatly appreciate the info and overall I’m leaning into the fact that I need to get a different locker this winter and give it another go around.

[1LE Fanatic]

I forgot to add in that the only playing I do with the car is autocrossing. I don’t drag race but I do enjoy spirited drives every now and again. The car is on fairly sticky tires Mickey Thompson’s 305/35 r20’s I believe. No suspension mods either. I more or less enjoy the cornering ability of the car and then full throttle out of corners so definitely no clutch drops or anything of that nature. Especially knowing that I’m still getting metal shavings/antisieze paste.

[Machh5]

Did you check and see if the carrier bearing races might be spinning in the caps?

[Aqua Blue RS/SS]

Originally Posted by 1LE Fanatic

I forgot to add in that the only playing I do with the car is autocrossing. I don’t drag race but I do enjoy spirited drives every now and again. The car is on fairly sticky tires Mickey Thompson’s 305/35 r20’s I believe. No suspension mods either. I more or less enjoy the cornering ability of the car and then full throttle out of corners so definitely no clutch drops or anything of that nature. Especially knowing that I’m still getting metal shavings/antisieze paste.

What you’re using the car for can be a pretty good stress test on the locker, you might be on to something there.

[hudson773]

[QUOTE=Greenhornet2;10905656]I am not sure you mechanical background but there is a LOT to rebuilding a diff. It is not something easy and there are more measurements than just backlash. pinion depth is overlooked a lot with novice rebuilders. Proper torque of the pinion nut to get the correct amount of preload is often overlooked.




as a mechanic for 50+ years I can't agree more. there is an art and much experience needed to do this operation. I have seen very recently a diff done by a so called reputable shop done for a friend on his corvette that so far has a distinct whine. someone didn't do something right. it takes many steps and many RE-STEPS to get it right. I don't think most places today have the experience in doing it correctly. I was taught by a very wise mechanic that there is a CORRECT way, an EXPEDIENT way and a WRONG way. for the sake of profit I think most places choose the expedient way.

[TheFlyingBananaSS]

[QUOTE=hudson773;10906460]

Originally Posted by Greenhornet2

I am not sure you mechanical background but there is a LOT to rebuilding a diff. It is not something easy and there are more measurements than just backlash. pinion depth is overlooked a lot with novice rebuilders. Proper torque of the pinion nut to get the correct amount of preload is often overlooked.




as a mechanic for 50+ years I can't agree more. there is an art and much experience needed to do this operation. I have seen very recently a diff done by a so called reputable shop done for a friend on his corvette that so far has a distinct whine. someone didn't do something right. it takes many steps and many RE-STEPS to get it right. I don't think most places today have the experience in doing it correctly. I was taught by a very wise mechanic that there is a CORRECT way, an EXPEDIENT way and a WRONG way. for the sake of profit I think most places choose the expedient way.

I couldn't agree more with this. Example...... a ton of people had trouble with the Richmond 3.91 gears. Many complaints of whining noises and gear teeth shearing off.

I had the Richmond 3.91 gears installed by a shop near me. Guess what? No whining at all. Completely silent! Solid as can be on the drag strip with around 700 crank hp. It's an art to setup a rear properly. Sometimes the extra 100 or 2 is well worth it.

[1LE Fanatic]

as a mechanic for 50+ years I can't agree more. there is an art and much experience needed to do this operation. I have seen very recently a diff done by a so called reputable shop done for a friend on his corvette that so far has a distinct whine. someone didn't do something right. it takes many steps and many RE-STEPS to get it right. I don't think most places today have the experience in doing it correctly. I was taught by a very wise mechanic that there is a CORRECT way, an EXPEDIENT way and a WRONG way. for the sake of profit I think most places choose the expedient way.

I realize shops tend to be expedient and not detailed oriented that's why I do everything myself. I have the time that shops don't have and I enjoy working and learning new things which is enough for me to justify doing it. Since you have 50 years of experience could you tell me what the differences are between the CORRECT way and the WRONG way? The more detailed you can make it the better off I will be this winter when I got to rebuild it.

[1LE Fanatic]

Did you check and see if the carrier bearing races might be spinning in the caps?

I have not taken the main bearing caps off since the initial rebuild therefore I cannot be certain. When I had it open though there was no play in the bearings that you could feel by hand. Being that the caps are aluminum as well as the casing I would imagine if the bearings were spinning in the caps it would have worn the aluminum out extremely quickly. I will double check when I tear it down this winter.