Rear Differential Specifications/Installing Limited Slip - 1996 RMW

Hi all--

I have a 1996 RMW with rear differential code GW9. I want to install a limited slip differential with 3.73 gears but I want to confirm my current specs to make sure I'm buying the correct LSD unit and gears. A local Buick dealer ran the VIN and told me I currently have a 2.93 ratio with an 8.5" diameter ring gear, but he was unable to tell me if if the pinion was 28- or 30-spline. Does anyone know the answer to this?

Also, does anyone have recommendation on a posi unit? Right now I'm looking at these Detroit Truetrac's by Eaton, or maybe a Yukon. My LT1 is pushing 337 hp/390 ft-lb. I'd like to add some snap coming out of the hole, but (if possible) I'd like to keep the noise level where it's currently at. Any guidance would be appreciated. Thanks,

Phil

30 spline axles.

On limited slip differentials, for all the whining about the "Eaton" style clutch type posi's, they're very simple to rebuild and tune. Whether Eaton, Yukon or whoever, heck, even Summit and Jegs make knockoffs for under $300 that take the same clutches. I can rebuild yours for cheap too if shipping isn't prohibitive.

Based on my experience, you can re-shim them at least 4 times before you have to replace the clutches, and I got around 40k miles on it before I even had to reshim the first time. Shim kits are cheap. If you're lazy, you could probably re-shim without even removing the differential from the housing.

You can also adjust the initial bite by changing the preload springs (or leaving them out entirely, and then the clutches only engage when the spider gears turn). My car's not on the road yet but I plan on trying this out as it'll be a 100% street car.

It's not the pinion that's 30 spline, it's the axles. The pinion is held on by a nut. You need a 3 series carrier, 30 spline 8.5 posi. A master install kit. A 3.73 ABS reluctor (custom now, since the OEM is no longer available), and a PCM reprogram.

You don't need the dealer to run info for you, all your wagon info is on your SPID sticker on the right side of your tailgate. Since you already have GW9 which is part of the trailer package, you may very well already have factory posi, option G80. Worth a check before ordering a new one. New 3.73s will go right on that G80 if it's in there...

UrbanPhil wrote:

Hi all--

I have a 1996 RMW with rear differential code GW9.  I want to install a limited slip differential with 3.73 gears but I want to confirm my current specs to make sure I'm buying the correct LSD unit and gears.  A local Buick dealer ran the VIN and told me I currently have a 2.93 ratio with an 8.5" diameter ring gear, but he was unable to tell me if if the pinion was 28- or 30-spline.  Does anyone know the answer to this?  

Also, does anyone have recommendation on a posi unit?  Right now I'm looking at these Detroit Truetrac's by Eaton, or maybe a Yukon.  My LT1 is pushing 337 hp/390 ft-lb.  I'd like to add some snap coming out of the hole, but (if possible) I'd like to keep the noise level where it's currently at.  Any guidance would be appreciated.  Thanks,

Phil

"I'd like to add some snap coming out of the hole" That will do it.
I added some snap by advancing the stock optispark.

Essentially it is still a distributor, meaning if you can twist it against rotation, the spark will advance. Now people may tell you the knock sensor will wipe out any gains, by then retarding the spark again, but not my experience!
I added a significant amount of hp seat of the pants feeling to mine.
What I did was grind the mount holes oval enough to turn the distributor maybe 3/16.
Easy to do and plenty of mounting strength left in the arms.
You can also buy a new designed  optispark that lets you twist it to do the same idea.

No knocking, running on regular gas. I have the tow pack, 1994 car, and before I did this the car felt lazy.

I will upgrading our 96 RMW this winter from the 2.56 to 3.08 with a Truetrac limited slip. I have gotten away from using the clutch type limited slips here in Minnesota unless it is a fair weather only vehicle and Buick is used as an occasional driver in the winter when I need to haul stuff and it isn't salty. Wicked on the ice or other slippery surface with any rear drive that always has some power to each side. The GM option G80 lockers work good if you don't beat on them too hard, but once you get to the 10.5, even they get pretty tough. Much better in slippery conditions. I have never had a Truetrac, so it will be interesting to see how well that works out for us.

Since I need a carrier anyway, I am just buying one of the gear change and Truetrac kits that come with all the parts including bearings, except the axle shaft bearing and seals.

booster, just leave the springs out of the center of the eaton-style and it'll behave exactly how you want.

sherlock9c1 wrote:

booster, just leave the springs out of the center of the eaton-style and it'll behave exactly how you want.

Don't know about that, never tries it. Adjusted pressures, yes but not no springs or shims.

As long as I have to get a carrier and and a limited slip anyway, I might just as well get one that is designed to work that way and has no clutches to wear.

Booster, the factory G80 in our cars is a cone type Auburn, not the Gov-Lock that the G80 gets in pickups.

jayoldschool wrote:

Booster, the factory G80 in our cars is a cone type Auburn, not the Gov-Lock that the G80 gets in pickups.

I wonder why they did that is one is available for the 8.5" axles? Gov-lock are much safer for everyday, everywhere drivers, I think.

Use a .038-.042 pinion shim, and you will only have to do it once. You can put the pinion in the freezer for a couple of hours, and the the bearing in a frying pan on high for 5 minutes. Get the pinion out of the freezer setting it on the gear end, put the shim on it, and using a kitchen mitt, drop the bearing on the pinion shaft. It will make a "chink" sound, and it is installed. If you want to, put a little lube on the shaft before you put it in the freezer.

The helical gear types are around $520 as of November 2020 at Summit. Jegs sells a clutch-type one for $200 cheaper. Jegs brand eaton-style posi.

The thing about the helical gear types is that the bias ratio is fixed. There is no adjustability on it. On a clutch-type posi you can not only adjust the preload via the spring rate, but also the amount of resistance by how tight you set the shim packs. The shim packs are less than $30 plus shipping from Summit, so even with that you're still far cheaper than a helical gear style posi.

Thanks all.  This is a lot of great information.  I'm showing my ignorance a bit re: the splines but that makes more sense now.  Jay--I checked and unfortunately I don't have a G80, so I'll have to start checking items off your shopping list.  

Here's my parts list so far (derived from Jay's list):

3 Series Carrier, 30 spline 8.5 Posi--I'm not 100% how to know if I'm looking at a series 3 carrier but do these units fit the bill?:
https://www.summitracing.com/parts/etn-19559-010
https://www.summitracing.com/parts/dtl-913a481
https://www.jegs.com/i/JEGS/555/62808/10002/-1 (This is the $399 JEGS unit posted by Sherlock)

Master Install Kit--I'm not sure about this one.  Does this hardware generally come with the ring and pinion set?  Do the shims come with it?

3.73 Ring and Pinion Gears (8.5")

3.73 ABS reluctor--These seems to be available from various outlets for around $100. For example:  
https://threepedals.com/products/brake/abs-reluctor-ring/abs-reluctor-for-3-73-ring-pinion-18-tooth/

PCM Tune

That's what I have so far.  Please let me know if I've left anything out.

Lastly, what is the difference (in performance) between the clutch-type and helical-type gears?

sherlock9c1 wrote:

The helical gear types are around $520 as of November 2020 at Summit.  Jegs sells a clutch-type one for $200 cheaper.  Jegs brand eaton-style posi.  

The thing about the helical gear types is that the bias ratio is fixed. There is no adjustability on it. On a clutch-type posi you can not only adjust the preload via the spring rate, but also the amount of resistance by how tight you set the shim packs. The shim packs are less than $30 plus shipping from Summit, so even with that you're still far cheaper than a helical gear style posi.

I have no problem with what you say on that at all. But what I am saying is that for my street only application that may see some slippery street conditions, I prefer to go with a unit with the preset, unwearing, properties that suit my needs. All this assumes that the Eaton unit does as it says, of course, and really operates with no power transfer until there is the right amount of wheel speed difference to get you out of a stuck condition or away from a stop sign. I like Gov-lock on the street with now moved up to 200 rpm difference for lockup, but they aren't available as aftermarket and in the small sizes do have some durability issues, even though they do hold up pretty well in basic street driving.

The kit I am thinking about is a bit under $700 for everything to do the job except the axle bearings. Maybe an extra $20 if I want pinion spacers instead of crush sleeve. The little bit extra it costs of going the absolute lowest cost way is not as important to me as the convenience and long term trouble free operation are. The goal is to get the car to the point of not needing any more major work in my driving lifetime (I am 71). $200 difference even is pretty small when I look at the whole project and the time to do it, especially with the engine where $200 will barely buy a set of rockers.

All of the posis above will fit the axle. The difference between the 2 and 3 series is that the 2 series is for the 2.56 gears, and 3 series is for all other ratio gear sets. The Truetrac requires a larger set of carrier bearings than stock. I do not know about the others, but the seller will know, and direct you to correct parts. I have the Truetrac in my axle, and am happy with it. Most of them are now made in China.

When you are replacing the differential, make sure the axles are not showing any wear, or you should replace them at the same time, with bearings. I got a set of axles from Moser, and they are about the same price as other "stock" axles, Check with Yukon, for axles too.

Make sure you have the tools you need before digging into the rear. You need a micrometer, dial gauge, magnetic stand, yoke holding tool (available from JEGS) https://www.jegs.com/i/JEGS/555/81656/10002/-1), lb in torque wrench, lb ft torque wrench, seal drivers, bearing race drivers, seal puller, large and small sledge hammer, and a pipe or rods to remove the assortment of bearings and races. I used a long piece of pipe to remove axle bearings, and a small piece of steel rod to remove the pinion races. I also got a pair of 2 ft pipes that fit over the yoke holding tool, and the end of a 1/2 in drive breaker bar (ratcheting one is preferred) because crushing the sleeve takes a lot of torque. I am not a big guy, and I had to put one against my belly, and grab the other with both hands to tighten it. A really long handle (36 inch) ratcheting breaker bar would suffice instead of one of the pipes.

When inserting the pinion seal, you must first put the bearing in, then the seal. Press the seal until it is even with the end of the housing. You can press it too far, and it will expose more of the pinion yoke than is necessary. You also need to put some sealant on the pinion spines, or you will have a slow leak from the yoke.

When you are adjusting the pinion rotating preload torque, "sneak up" on it a little at a time after taking the play out of the bearings. 20-35 INCH pounds of rotating torque is by the book. Sometimes an old style bar torque wrench can give you a better reading than a break away torque wrench. Attach it to the pinion nut, and read it while swinging it in a circle.

We put a Eaton TrueTrac in the Wife's 454SS to replace the factory G80 GovLock. It works great! It seems to almost always have both wheels locked, just like a clutch/cone style posi.

It's 2020, messing around with a standard crush sleeve is OVER. Don't even have to go with a solid spacer either.

https://www.ratechmfg.com/smart%20sleeve.htm

94Woody wrote:

It's 2020, messing around with a standard crush sleeve is OVER. Don't even have to go with a solid spacer either.
https://www.ratechmfg.com/smart%20sleeve.htm

Absolutely. The original design was dumb. This is so much better.

94Woody wrote:

It's 2020, messing around with a standard crush sleeve is OVER. Don't even have to go with a solid spacer either.

https://www.ratechmfg.com/smart%20sleeve.htm

Looks like a good replacement. I never came across it in my searches. It should make it easier to get the proper preload torque setting for the pinion bearings. Which one fits the GM 8.5" rear?

Fred Kiehl wrote:

94Woody wrote:

It's 2020, messing around with a standard crush sleeve is OVER. Don't even have to go with a solid spacer either.

https://www.ratechmfg.com/smart%20sleeve.htm

Looks like a good replacement. I never came across it in my searches. It should make it easier to get the proper preload torque setting for the pinion bearings. Which one fits the GM 8.5" rear?

11002

https://www.ratechmfg.com/gmstandard.htm

Thanks Fred and everyone for the detailed rundown. Does it make more sense to try to find a G80 wagon axle and swap the whole thing out? Would it have to be a G80 Caprice or Roadmaster wagon axle?

Also, is there some reason I can't (or shouldn't) switch my 2.93 out for a used/remanufactured G80 LSD unit like this one?:

https://www.ebay.com/itm/GMC-Chevy-8-5-10-Bolt-G80-Eaton-Posi-30-Spline-Gov-Lok-LSD-3-73-Truck-Suburban-J/233056835164?hash=item364345165c:g:h10AAOSwSVdbw9vO

In my opinion, the truck mechanical GovLock is a terrible unit and prone to breaking.

The G80 in the trucks is different then the posi unit used in our wagons?

Yes. It's a mechanical locker, and is easy to break. Its nickname is GovBomb. The rear end is open, then one tire spins, then locker engages to drive both wheels. Good for mud. Bad for street car launches. Don't install one.

OK TY Jay and Joel. That's good to know. Theoretically if I found a complete G80 posi axle from a B-body wagon, I could swap the whole thing right? Am I restricted to wagon axles only if I go that route? (as opposed to a RM or Caprice sedan for example). Are the stock G80 wagon rear-ends stout enough to handle the extra power? So many questions...

The stock G80 will work for a while, if it is still good. You can put the sedan axles under a wagon, but it will have some alignment issues with springs, shocks, and lower arms. Bushings will be stressed, and the springs, and shocks will be cocked inward at the bottom. I would not do it. I do not like to bastardize things. I would rebuild the rear before I would put another one, especially a sedan unit in my car. Buying a used posi is a crap shoot at best. I hate to do things twice, and saving the money is not worth the risk.

If you are intimidated by building a rear, there are a number of us on here who have done it, and can walk you through any issues you might have. I made all of the petty mistakes, and mine works great. I am pushing about 450hp, and 500+ lb ft of torque through it with a 454.

Don't buy some used diff and toss it under your car. After decades of use most factory posi units are long dead anyway.

Just get the parts and build what you already have.

I would have to agree with Fred. Assuming the G80 (Gov-loc) is actually rebuilt well and all the fragile lockup pauls and such replaced, it would last fairly well on the street if not abused. I would not just put in a used one.

If you don't mind a clutch type posi, they are cheap to buy. For a bit more you can get a Tru-trac that would be more similar to a Gov-loc in feel bit more durable over time, or lots of other options.

The factory limited slip differentials are not rebuildable. They are an Auburn cone-style design. And GM pressed Auburn Gear to cheapen them even more so that they are not even exchangeable as the Auburn Pro series differentials are (through Auburn). Once they go, they go. That said, they seemed to last a lot longer in the Buicks than in the Impalas and Caprice police cars (gee I wonder why). I have one here on the bench that still works (it's already earmarked for a particular car).

If you go to any other gearset, you must buy gears that are machined for the ABS reluctor unique to these cars. And you must buy the ABS reluctor with the right number of teeth. The Impala SS forum has all the info needed.

Personally I thought the 2.93 axle choice was fine for these cars (having owned four of them), although if you are city driving only, 3.73 is the ticket (I put these into my Caprice 9C1 and it was a hoot around town). As to a whole axle swap, on a rusty car it will be some work. If you have a shop willing to install it, a complete axle swap is a viable route. Most desirable is the 91-93 3.23 posi rear end. If you can find one of those, I would seriously consider swapping the whole axle.

I installed a universal ABS reluctor.

94Woody wrote:

I installed a universal ABS reluctor.

I did something similar, but I have a lot less space, so it is carefully formed.

OK, thank you all. The consensus seems to be: build it from scratch, so that's what I'll do. Mainly I was concerned this job is above my pay grade (and perhaps beyond the limits of my current shop) But it sounds like things could get equally hairy with an axle swap. I don't want to go through all that trouble only to have the thing grenade on me. I'll start getting my parts together. I appreciate all the help from the longroof community. Everyone have a great Thanksgiving and stay safe. Best,

Phil

Hi Everyone,

I hope you had a good Thanksgiving.  In lieu of online x-mas shopping I ordered up some of these rear diff components today.  Here's what I ended up going with:

Rear Differential:
https://www.jegs.com/i/Detroit+TrueTrac/363/913A481/10002/-1?utm_source=criteo&utm_medium=cpc&utm_campaign=retargeting

Install Kit:
https://www.jegs.com/i/JEGS/555/61226/10002/-1?trk_msg=0T0QTD5238MKF33E6C70O4HR8S&trk_contact=5UR4SGSNKR1NNKLK9F896LNQ9G&trk_module=tra&trk_sid=MKM96B8N6DOE864G4G8E1O10L8&utm_source=listrak&utm_medium=email&utm_term=Product&utm_campaign=Transactional&utm_content=Order+Confirmation

3.73 Ring & Pinion:
https://www.jegs.com/i/JEGS/555/60031/10002/-1?trk_msg=0T0QTD5238MKF33E6C70O4HR8S&trk_contact=5UR4SGSNKR1NNKLK9F896LNQ9G&trk_module=tra&trk_sid=MKM96B8N6DOE864G4G8E1O10L8&utm_source=listrak&utm_medium=email&utm_term=Product&utm_campaign=Transactional&utm_content=Order+Confirmation

I called Jegs to confirm I was getting the correct install kit.  The guy seemed pretty sure of these components but if you notice that I made an obvious mistake please LMK! As far as I can tell I still need the reluctor and maybe the new style crush sleeve but I'm making progress.  Thanks again for all the help!

Hi all. My progress on this has been glacial but I'm trying to get back to it. I have a couple quick questions:

1. Is this the right reluctor for what I'm doing?:
https://threepedals.com/products/brake/abs-reluctor-ring/abs-reluctor-for-3-73-ring-pinion-18-tooth/

2. Which of these Ratech smart crush sleeves is the right one for this application?:
https://www.ratechmfg.com/smart%20sleeve.htm

Thanks. I hope everyone is well.

I hate to say this now, but the Jegs 555-60031 gears you posted above specifically says "Does not work with pinion-mounted ABS sensor". I haven't seen a B-body pinion in awhile so I don't know if this is actually true or not.

The reluctor you posted is the correct one.

Crush sleeve; 11002.  1970-1997 GM 8.5" passenger car. The chart is here.

BTW that Ratech pinion 18001 holding tool is great. I wish I had the 18002 tool but it wasn't available when I bought the 18001.

Welp...that's a bummer. Thank you for catching that Sherlock. I missed it and the reluctor issue never came up in my conversation with the Jegs guy. Would you be able to tell by looking at the pinion whether it's compatible? What are my options if it's not? The saga continues...

Your option is to return it, and get one that works with the reluctor.

I called Jegs about this issue. The guy is now telling me the gears I got WILL/SHOULD work with the pinion-mounted reluctor in the 1996 RMW (but it is listed--in his computer--as incompatible with the 1990-1996 Impala/Impala SS/Caprice). I'm not sure whether to trust this advice. If I have the reluctor and the pinion gear in my hands will I be able to tell if they're compatible?

I would not trust the salesman. You can order a set that will work with the reluctor. Why take a chance, when you can get one that is guaranteed to work. If it does not say it will work, do not buy it, because it will not work.

you need to make sure that the person youre talking to actually knows what they are talking about.

case in point, tried to find a used speedometer for my 89 R series crew cab, the speedometer is almost identical to the same year suburbans and the other square body gm trucks and SUV's but it has the 5 digit odometer with a trip gauge, had a local shop look for one and they said they found one, go in and they try to give me a whole instrument cluster out of a 90's C/K series GM truck claiming that it's the right dash for the truck based on the VIN, until I told them to show me how it was supposed to fit, then they couldn't think of how the place they got it from messed it up. I told them the place probably only looked at the year and make, and didn't even bother to see if there was a difference between an r series truck and a C series.

Another time I looked for a new grill for my 87 Pontiac Safari wagon, and got tons of results, but when I looked more closely at the fitment I also saw that all of them would also fit a 1987 GMC Safari, which as members here should know is impossible as the GMC safari is a minivan not a wagon.

Hi All.  I'm resurrecting this old thread to give you an update on the rear diff project and say thanks to everyone for your help.  Life got busy and I ultimately opted to hire a local transmission shop to install my parts. They did an awesome job and had the car back to me in 3 days. (Continental Transmission in West Seneca, NY)  I've been driving it for several months with the 3.73's and I'm extremely happy with the results.  The car is much more responsive and fun to drive around town.  My fears about highway driveability were largely unfounded.  In case anyone is looking to do a similar project, here's my parts list with links and some notes:

1. Detroit Truetrac Differential GM 10-Bolt (Part Number: 363-913A481)

https://www.jegs.com/i/Detroit-TrueTrac/363/913A481/10002/-1?trk_msg=SQ5AS03GVL9K148P6L2HN7U9MO&trk_contact=5UR4SGSNKR1NNKLK9F896LNQ9G&trk_module=tra&trk_sid=1OF1SCQLOE93FM8KHVEFAO4UE4&utm_source=listrak&utm_medium=email&utm_term=Product&utm_campaign=Transactional&utm_content=Shipping+Confirmation

2.  Complete Differential Installation Kit GM 8.5" (Part Number: 555-61226)

https://www.jegs.com/i/JEGS/555/61226/10002/-1?trk_msg=0T0QTD5238MKF33E6C70O4HR8S&trk_contact=5UR4SGSNKR1NNKLK9F896LNQ9G&trk_module=tra&trk_sid=MKM96B8N6DOE864G4G8E1O10L8&utm_source=listrak&utm_medium=email&utm_term=Product&utm_campaign=Transactional&utm_content=Order+Confirmation

NOTE: The bearings and races in this kit were incorrect for this application.  I confirmed this (and got the correct part numbers) by calling Eaton.  Jegs sent me the correct bearings no questions asked.  Of course now I can't find the part numbers...It might be these (Part No. 873-D1585), but I'm not 100% on that:

https://www.jegs.com/i/Strange-Engineering/873/D1585/10002/-1

3.  Yukon Gear & Axle YG GM8.5-373 - Yukon Gear & Axle Ring and Pinion Sets (Part Number: YGA-24158)

https://www.summitracing.com/parts/YGA-24158

NOTE: If you have a pinion-mounted ABS reluctor, make sure your gearset is compatible with that setup.  As you recall, the original gears I bought were not compatible and I had to swap them out for the Yukons.

4.  Fel-Pro Differential Gasket (Part Number: FEL RDS55028-1)

5.  ABS Reluctor for 3.73 ring and pinion (18 tooth) (Part Number: 2314-40/3018)

https://threepedals.com/products/brake/abs-reluctor-ring/abs-reluctor-for-3-73-ring-pinion-18-tooth/

6.  Ratech Smart Sleeve Crush Sleeve (Part Number: 11002, Replaces 3103, 3109 & 4109)

https://www.ratechmfg.com/smart%20sleeve.htm

I think that's it (+ gear oil); hopefully this is helpful to someone.  The motor is turning around 2100 RPM at highway speeds (65-70 mph) which is where I thought it would land.  It's perhaps a little louder on the highway but not much. (3.73 is about as high as I'd want to go with my setup) As someone pointed out, these cars coast so nicely you can just let momentum do the work at highway speed.  I didn't get a tune after doing this.  I'm not sure if that's recommended or necessary. The car was running great so I left it alone.  My aim is to supercharge this LT1 so I'll be seeing the tuner again in the future anyway.

One issue I still need to address is the speedometer, which now reads about 20 mph high.  Anyone know how to fix this?  I'm not sure if it's a mechanical or computer issue (or both?)  Please LMK if anyone has questions about the project. Thanks again for all the helpful advice.  I hope everyone's having a great summer.  Best,

Phil

You need to tune the speedo in the computer and that will also reprogram the shift points for the gear change. I am not certain if you can do just the speedo and not have it change the shiftpoints.

I just did ours for the 2.56 to 3.23 change and it moved the shift points, it appears, but I haven't compared in the actual program, to shift at the same rpm it used to which reduces the speed it shifts at.

The program for shiftpoints is set by speed, not rpm, so that makes sense for it to do. Your shift points would have stayed at the same speed with a bit of variation because of possible vacuum and TPS changes due to the gears so you would have higher rpm shift points at the amount of the gear change. This certainly would add to the feel of power and responsiveness.