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TxBuicks

1991 Roadmaster Wagon - Ring and Pinion Gear Replacement

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My 1991 Roadmaster wagon broke last week.  Wouldn't go forward or backward.  It wasn't long before I realized the problem.  I saw the drive shaft spinning and the rear wheels not spinning so I know it had something to do with the rear end gears.  I pulled out the differential assembly yesterday and, yep, the ring gear is almost toothless, and the pinion gear is too.  My question is, how does the pinion gear come out?  I know I have to remove the U-Joint and the yoke from the front of the differential, but then does the pinion gear just push out through the back end of the differential housing or is it pressed in?  This is my first attempt at rear end gears so I want to know what I'm getting into.  And the shop manual just says 'remove the pinion gear and shaft' but it doesn't say how.  And I'm thinking the big bolt that holds the yoke on is going to be fun now that I have the differential gears out there won't be any resistance keeping it from spinning when I try to take it loose.  Probably have to jam the yoke somehow while I take the bolt loose.

 

Any thoughts on any of this?

 

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There is a nut on the front (outside) of the pinion under the rear half of the U joint.

With the nut removed, the rear half of the U joint comes off (it might take a puller) 

Then the pinion should slide out the back (pretty sure the differential assembly must be out)

When reinstalling there is a "crush sleeve" that is compressed to get the proper tooth engagement.   If the tighen the nut too much, you must start over with another crush sleeve...might want

to get help for that part.

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I'd like to see pictures of that.   And how much noise was it making?  If that is a common GM differential, a used part might be considered.

Willie

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It wasn't making any noise that I could hear.  It just broke suddenly.  I have the differential apart and the teeth of the ring gear and pinion gear are worn down and rough looking. The are definitely chewed up. It was completely dry, so I guess the oil must have leaked out slowly over the years and I honestly never checked it.  Lesson learned. 

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Edited by TxBuicks (see edit history)

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Should be common differential with Chevrolet cars of the day.  Would try one of the self serve salvage yards for a used one.

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The rear axle on the B-car WAGONS is a wider-track rear axle than the sedans, but I suspect the guts are common. The rear ratio "tooth count" should be stamped on the outer edge of the ring gear, as "41-12", for example . . . that's the gear ratio.

As mentioned, when the differential yoke nut is removed, the yoke will separate from the splined end of the pinion gear. Then the pinion should slide out of the inner and outer pinion shaft bearings, plus the crush sleeve.

On some of the newer gear sets and vehicles, they want you to measure the torque it takes to break the pinion nut loose. Then, rather than a crush sleeve, you use that dimension to build "a shim stack" to use in place of the crush sleeve. Then use the "removal torque" to torque the new set-up together.

ALSO be advised, the pinion nut torque can be GREATER than you might be expecting. Some vehicles need a double-air line adapter for the air wrench to get the necessary torque to get the nut off and on. So, be sure to check the torque specs in the service manual (or applicable GM Unit Repair Manual . . . where much of the "assemblies" repairs are detailed).

In addition to the inner and outer pinion shaft bearings (plus seal), with that much metal in the housing, you'll also need the two differential side bearings! And, for good measure, you might want to also consider the axle shaft "wheel bearings" and seals, too, as they receive oil during cornering and such from the main differential sump. If the rear end gears were that dry, the same could be true for the axle shaft bearings! IF you find any wear on the axle shaft bearing surfaces, you'll need new axle shafts too!

In doing a rear differential gear replacement, how the gears are "set-up" is important. That pattern the pinion gear puts on the teeth of the ring gear, to be specific. This is where the pinion and differential side bearing shims become important. I hope that you kept the existing side bearing shims all together and separated as to which side is which.

Worst case scenario, you might want to fine a used assembly from a salvage yard. The B-car wagons were not that plentiful, but the same parts should fit Chevy, Buick, and Olds B-car wagons. I also suspect that the rear axle ratio would be the same among all of them, unless there might have been a trailer tow optional (lower) ratio. But you'll want to pull the rear cover and roll the ring gear around for inspection purposes and can also check the stamping on the edge of the ring gear at the same time (for ratio verification information).

Usually, there is another method of identifying the rear axle ratio on many GM rear axles. About 1/2 way between the differential section and the rear wheel bearing, on the passenger side of the axle tube, usually positioned where you would be looking at it if the car was on an overhead lift . . . is a series of stampings which is the "birth certificate" for the rear axle assembly. Type of axle, plant it was built in, date it was built, AND the last three letters will identify what's inside . . . the gear ratio and whether it is a "limited slip" (i.e., PosiTraction) differential are denoted by the last three letters in that string of identification. With those last three letters, decoded with the applicable GM parts book (or possibly an online information source), you know what's in there.

If you get a rear axle gear set from GM, it could be past $500.00 by itself. About $65+ for each bearing (2 pinion bearings, 2 side bearings, 2 outer axle shaft bearings), plus the seals and rear cover gasket. With labor and lube and incidentals, you could probably at another $500.00 easily.

If you get an assembly from a salvage yard, you might want to exchange the rear brakes on it for the ones on your vehicle, so you'll know what's under there, at least, or do a rear brake job as a matter of course. Just do your due diligence in that search! Just remember, "It's used" and they'll probably want "an exchange" for the best price.

Considering what it would take to repair your axle assembly (stripping, degreasing, and pressure washing the innards of the axle housing, for example, and then completely cleaning and re-installing the differential case and spider gears, plus the other needed new parts and getting it all assembled and adjusted correctly), finding a good unit from the salvage yard might be the best option. Provided a good one can be found.

Not that a private individual couldn't do all of these things, provided the resultant hazardous waste fluids were collected and disposed of properly, plus a good bit of brake cleaner (unless there is a handy cleaning parts washer nearby), plus a dial indicator and some "gear marking compound" to ensure a good pattern is on the ring gear teeth (or the Kent-Moore rear axle set-up special tool kit), it's all "your time and money". Not to forget the HD air ratchet necessary for the higher torque needed to reove and install the pinion nut (AND an air compressor powerful enough to run it!).

In the '80s and prior, it seemed that only "special people" in the hot rodding realm of things could do a ring/pinion gear change and have it not hummmm when done. Many general mechanics didn't want to do rear axle gear changes, either. So you had to ask around to see who could and would do them. Most of these guys were serious drag racers and usually did good work. At the dealership level, most of the heavy line guys could do rear axle gear replacements, but with the help of the Kent-Moore (now SPX) GM Essential Tool Kit that seemed to make it much easier to do and do it right.

Sorry for the length, but this is not quite like changing a carburetor or alternator.

As for rear axle lube, that car should spec the normal rear axle lube as OEM. The newer GM rear axles (as in light trucks) spec a synthetic lube now. Of course, synthetic lubes are available in other brands, but the normal stuff should be fine for your uses.

Please keep us advised of your progress.

NTX5467

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The pinion bearing in one of the Roadmasters I owner went bad. I just bought a used rear end and replaced the whole thing, drum to drum.

Most of the trailer package cars have posi. They are the cars with a mechanical fan. As I remember trailer packages had something like a 2.80 gear set and the peg leg cars have a 2.72.

 

Just replacing the pinion gear is about a $400 and the few people I know whom have had the job done complained about howling (the rear end and them) or other noises. My preference would be a used rear end and hiring a couple of yahoos to come over and put it in under my direction, just grunt work.

Bernie

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You might be a Yahoo if you can be predicted to be found at a gas station/convenient store around lunch time. I live on the edge of town and there is one less than a quarter mile from me. In the last year or so I have needed to move my wife's drier (she doesn't have a collector car) from the basement to her new laundry room and unload a vertical 80 gallon air compressor from my truck.

All I have to do is wait until noon and go up to the corner with a $20 bill. I wait no more than five minutes and I can draw a very accurate  picture of the two guys and their vehicle ahead of time. It sure is better than doing the heavy stuff yourself.

 

On the whole rear end rebuild thing; in 1972 or 1973 when I was "less innocent" I had a set of 4.11 gears for my '58 Chevy wagon that I left with a "mechanic" to install. That's back when we called a rear end a ten bolt because ten bolts held the ring gear on. (Counting the number of bolts that held the corner on never seemed to carry a depth of knowledge to me.) That was the set that required a spacer and long bolts, which I provided. After a month I liberated it with the gears installed, no spacer, bolts in a bag, and lots of slop. The old juicer told me the bearings needed adjustment. I had read the book, looked at the pictures or bluing and dial indicators, and figured it was complicated.

 

Even at 24 years old I knew if they locked us in a room with boxes on the floor, there was no way I was going to share the banana with this guy. I took it home and put the housing in my Great Grandfather's vice nose down and took all his work apart. I put it back together right and set it up by feel. "umm, that's a little tight, that's and little loose, and finally, just like Goldielocks, things were just right.

 

I ran that car hard for about a year, then sold it to one of the older brothers of a REAL Yahoo family south of town. He ran the crap out of it and the last time I saw it the engine was blown. I'd say that rear end held up pretty good. And it is one of those instances that molds to character of a youth. Yep, take it back from the so called professional and do it yourself good enough to be bullet proof against the Yahoo's. Now, where's that room with the boxes and banana?

Bernie

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The pinion bearing in one of the Roadmasters I owner went bad. I just bought a used rear end and replaced the whole thing, drum to drum.

Most of the trailer package cars have posi. They are the cars with a mechanical fan. As I remember trailer packages had something like a 2.80 gear set and the peg leg cars have a 2.72.

 

Just replacing the pinion gear is about a $400 and the few people I know whom have had the job done complained about howling (the rear end and them) or other noises. My preference would be a used rear end and hiring a couple of yahoos to come over and put it in under my direction, just grunt work.

Bernie

 

 

My type of repair.  :)

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The pinion bearing in one of the Roadmasters I owner went bad. I just bought a used rear end and replaced the whole thing, drum to drum.

Most of the trailer package cars have posi. They are the cars with a mechanical fan. As I remember trailer packages had something like a 2.80 gear set and the peg leg cars have a 2.72.

 

Just replacing the pinion gear is about a $400 and the few people I know whom have had the job done complained about howling (the rear end and them) or other noises. My preference would be a used rear end and hiring a couple of yahoos to come over and put it in under my direction, just grunt work.

Bernie

Pinion bearings are done separately, but from what I've seen getting a pinion gear without the matching ring gear might be a trick.

Once, a friend had a 396/375 4-speed '67 Chevelle, ordered with a 3.08 rear axle. Another friend figured out that the only difference between the 3.08 and a 4-series gear ratio was 1 tooth on the pinion gear. Another guy they shared a miniwarehouse shop with had a rear axle with THAT pinion gear in it. So . . . in a very clandestine operation, taking care to not disturb the dirt on the rear axle cover, the pinion gear was extracted and then later replaced with the one from the 3.08 gear set. Miraculously, it worked and was quiet. Never did know about what the other guy's reaction was as he moved out a month later. Such a deal!

NTX5467

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If you plan to tackle the job, I have a matched ring and pinion out of my '96 Impala SS (long ago sold) that I swapped 3.42's into when the car had about 30k miles on it. If not, I would agree a total swap would make sense as the easiest way to go. A couple of hours and you're done. There was a guy in New Hampshire, I believe, who sold 90's B-bodies and parts, mostly 9c1 and wagons. I couldn't find him on a quick search, but if I find a link I'll add it. Good luck!

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More progress yesterday tearing it all down.  Here are pictures of the pinion gear (what's left of it). I don't know how this thing ever ran after looking at this. I determined it is a limited slip setup (posi-traction), 3:23 ratio, 8.5 inches.  I found pieces of the posi-traction brackets and springs in the differential case.  My shop manual tells me the Auburn limited slip differential is not servicable and should be replaced as a complete unit.  Does anyone know about that?  I also posted a picture of what happens when you hold the slide hammer (used to remove the axle bearings) a little too close to the slide hammer.  One thing for sure, I'm learning as I go.

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What Ben said X2! That is hurting me just to look at it.

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I know it has been a while, but I'm finally making progress.  I decided that replacing the gear assembly with new ones would probably require more skill and knowledge and money than I had.  So I started calling salvage yards.  I was looking specifically for a rear end for a 1991-1993 Roadmaster wagon, nothing else seemed to fit without some modification.  With the help of Pete Philips, I found one in Bud's Salvage yard in Aline, OK that I could afford.  It was an exact match, having come out of a 1991 Roadmaster Wagon.  It is not a posi-trac unit like the one I had, but it will do.  I brought it home and took the cover off.  It looks brand new inside.  Clean oil, no gunk buildup, and the axles turned smoothly and quietly.  I decided not to replace the axle bearings and seals because that would require me to take apart some of the rear end gears to release the axles.  I didn't want to mess with any disassembly of this unit.  I spent several hours over a few days going over it, removing the emergency brake cables, cleaning, etc.  I then removed the old broken rear end from the wagon.  The new one is ready to go in.  With the help of some nice weather, perhaps I can get this done soon.  Due to my other work schedule, I can only work on it over the weekends.

 

In case you haven't noticed, this is the same 1991 Roadmaster Wagon featured in an article in the November, 2015 Bugle.  Patricia is excited to get her car back!

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Edited by TxBuicks (see edit history)

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I put the new rear end this weekend.  It was an exact fit, even the brake line clips were in the same place.  What a relief it is to get the exact same part.  All that's left now is the brakes, and putting the gas tank back in.  The hard part is over.  Weather permitting, I should be test driving it soon.

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Well, this has been a fun read. I bought a '91 OCC for my wife that I'll be going through soon, and it has a bit of a howl in the rear end. The previous owner included a new pinion bearing, but I'm not so sure I want to install it.

After seeing that photo of Tx's hand, I'm REALLY thinking that I'll let someone else overhaul the rearend for me...

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Matt, Don't let my stupidity determine your decision.  Just remember to hold the slide hammer on the handle and not the slide bar!  I'll tell you why I went with a rear end instead of trying to change the gears.  After talking to several knowledgeable gear people, I really didn't think I had the skills, patience, or tools to replace the gears. Apparently it takes a skill, usually developed from experience.  I simply didn't have that experience.  The manuals discuss tolerances and back lash and torques and use micrometers and dial gauges to get everything in place properly, and, honestly, it was a bit frightening to think about for me.  I am happy with the decision I made to find a replacement rear end. The key to my happiness is that I finally found one that was from the exact year and model as mine.  It wasn't from a similar GM vehicle.  Being an exact replacement made it so much easier.  Even the brake line clips were in the same place.  So, If I had to do it over again, based on my facilities, tools, and ability, I'd go with the rear end replacement, instead of messing with the gears.  Now, I'm sure there are some that will read this and disagree, but I'd bet you those guys have done the gear replacements several times and feel comfortable with it.  I didn't.

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You touched on a lot of important aspects, Roy.  It's not just a matter of throwing the parts together, and I'm going to be pretty demanding of the vehicle once it goes into service.  It will be a daily driver and will haul a 3500 lb trailer of a fairly regular basis (and I'll be putting down way more than the stock 170 hp through it).  I'm more inclined to pull my axle out and let an experienced shop go through the whole thing with a fine-tooth comb, as a salvage axle may leave me in the same situation 10,000 miles from now.  I want to do this once and not think about it again for at least 150,000 miles.

Edited by SpecialEducation (see edit history)

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I got it back together and drove it to work this morning. It has been about 8 months since it broke. A long time to sit, and one of my most ambitious projects. I'll drive it for about a week before I'm confident there won't be any more issues with it. No telling how long the rear end I pulled out of the salvage yard sat, but it appears to be happy.

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