How to Rebuild a GM 8.5 10bolt - HowTo Thread

[khan0165]

maybe it's just me, but I've had a hard time finding good trustable information on how to service or rebuild a rearend. There's lots of vague or general information on certain components, particularly axles, but no complete "How-To" threads.

For a no0b like me, it's been tricky. I've had to ask alot of questions and piece together alot of bits of information and I thought I would start a Build Thread on a complete "How-To" for this knowledge base.

This story started a couple years ago when I noticed alot of Posi slip in my 8.5 10bolt at the drag strip when doing a burnout. I've continue to beat on it since, and it has progressively gotten worse. I am now at the stage where I plan to dig into this thing over the summer.

I hope to do all the work myself. I work very slowly, so hopefully you guys will be able to follow this as it goes along.

cheers,
khan

 

[khan0165]

ok, last post was an intro... now let's get into the details.

the OEM GM 8.5 10bolt placed under 80s Regals and Grand Nationals uses an Eaton posi unit. Some units were not posi's, some other vehicles used Auburns. The Auburn units are not rebuildable, the Eatons are. I will be covering the rebuild of an Eaton Posi unit (which is what I've got in my GP).

Now since the 80s, Eaton has converted to a coil spring clutch pack... like this

but our OEM styles use an S-spring Clutch pack... like this

there is plenty of information, tech articles, and rebuild support for the new style. The S-spring type has become rare and GM stores have discontinued the Clutch and Spring parts for this style.

After a year of researching, I have tracked down all the parts required to rebuild my S-spring style posi, and I am now at the stage of ordering these to get ready for the rebuild over the coming summer.

stay tuned for next post where I'll list all the parts,

cheers,
Khan

 

[khan0165]

I thought I would post some pictures to show how it all works

above, you can see the components that sit inside the Differential Case, also known as the "Carrier". The Pre-Load Spring is what I described in my last post, they can be the Coil style like shown here, or the S style which I will cover. Around the spring are the Pinion Gears and Side Gears, but they are more commonly referred to as the "Spider Gears". Our differentials do not have a "Cone Clutch Assembly". The clutch packs in these units are a series of flat Ring shaped plates placed on the outside of the Side Gears inside the Carrier.

you can see the "square ears" of these clutches sticking out in a slot in the Carrier on either side of the Side Gears.

and they look like this (top right)

in the new style Eaton posi, with coil Pre-load Springs, these clutches have round "ears" and are not visible on the outside of the carrier.

The Pinion Shaft holds the Pinion Gear and Spring assembly central inside the Carrier. Together they mesh with the Side Gears and press up against the Clutch Pack. Together this is called the "Spider Gear Assembly" and is what creates the "Limited Slip" effect inside a Posi differential.

The axles mesh splines inside the Side Gears and are held in with a C-clips

The Ring gear is attached to the Carrier and driven the by Pinion Gear

I hope this explains the functions of a Limited Slip differential. The wear components are the Carrier Bearings that hold the Carrier inside the differential housing, the Axle bearings, Thrust Washers which that apply pressure to Pinion Gears inside the Carrier, Pinion Shaft Bearings and Seal. And new shims are required if components have changed and have to reset the Carrier inside the Housing.

stay tuned for next post where I post the parts I've selected for the rebuild,

cheers,
Khan

 

[Longroof79]

Mr Khan,
I commend you on the informative write up. I will be following it..

 

[khan0165]

thanks Jack! I hope it will be helpful for me as well. This originally started as a plan to replace clutches, but seeing how I am starting to race this car more and more, that plan has expanded to a full rebuild and going to larger 4.11 gears.

As a note for everyone, for GM 10bolts, there are Carrier series, 2 and 3 I believe? This dictates the size of Ring Gear you can fit inside the Carrier. I believe a 2nd Series Carrier can house 3.08s and lower, and a 3rd Series can house 3.23s and higher.

so after much searching, here's the list of components I've found:

- S-Spring: surprisingly as rare as the square ear'd Clutch Packs are, the S-Spring itself is quite commonly available. You won't find it in the more common Jegs or Summit, but alot of Drivetrain specialty stores carry it.
the cheapest I've found are at GbodyParts.com http://www.gbodyparts.com/product_info. ... 9cd8a8cd9b
and DTS (Drivetrain Specialists) http://www.gbodyparts.com/product_info. ... 9cd8a8cd9b

- Clutch Pack: the proper clutch pack (square ear'd version) has been discontinued from GM and after ALOT of searching, I've found it in two places.
after speaking with PontiacGP, we found them at http://www.iwerearendsonly.com/ , but they are Canadian and their prices are likely double what you'd find in the states.
the other place is Nitro-Gear.com IPKGM8.5-PC-E 8.5" GM S-SPRING POSI CLUTCH KIT, 2 SQUARE TABS
scroll down it is under part no. IPKGM8.5-PC-E 8.5" GM S-SPRING POSI CLUTCH KIT, 2 SQUARE TABS

- Thrust Washers: I was told the factory used a soft brass/copper material, so these wear out quickly. You can buy steel ones for replacement, which last much longer. Seems like I will have to get these from http://www.iwerearendsonly.com/ , but if anyone knows of a cheaper place, please let me know.


okay, now for the Rebuild Kit. I have three Options...
- Basic Kit (Shims, Gear Bolts, Marking Compound, Crush Sleeve, Pinion Seal & Nut, and Gasket & RTV) $19.99
- Complete Kit (includes all from Basic Kit, plus Carrier Bearings) $91.99
- Deluxe Kit (includes all from Basic and Complete Kit, plus Axle Bearings) $129.99

here's a link for the Jegs Deluxe Kit, with Timken Bearings http://www.jegs.com/i/JEGS-Performance- ... Id=1176762
scroll down to see a description of the Basic Kit and Complete Kit


so, evaluating the prices, $19.99 $91.99 and $129.99
what do I need? and what can I get by without? I've been told that Carrier Bearings almost never go bad, and there doesn't seem to be a problem with my Axle Bearings. Besides being slippy, my rearend works very well, very smooth, and holds up to alot of abuse.

... is it worth the extra $72 or $110 for bearings? Or can I reuse the bearings I have now?

thanks,
Khan

 

[1987GN]

Hello Khan,
I hope your still on this site. I'm located about 90 mins south east of you and would like to ask a few questions on how your rebuild went. I'm having some problems with my diff and not sure which way to go. Can you help me out?
Thanks. Stewart.

 

[pontiacgp]

Khan, I just rebuilt an Auburn today for a buddy who owns a shop. It was in a toyota truck and it has the S spring but the smaller rounded tabs on the clutch packs. Seems like Auburn switch the style of retention springs they use. My 67 Auburn has the 4 spring set up

 

[khan0165]

Hello Khan, I hope your still on this site. I'm located about 90 mins south east of you and would like to ask a few questions on how your rebuild went. I'm having some problems with my diff and not sure which way to go. Can you help me out? Thanks. Stewart.

hey Stewart,
yeah, I'm still kicking around the forum, winters are slow for me.
Unfortunately, I did not get around to working on the rear-end over the summer, like I had planned. I had the motor out for rebuild, and things went wrong, took up my whole summer.

There's a few things I need to update on this thread... After internet sourcing all the parts, I contacted the sites 1 by 1 to get quotes. It turns out no one had the S-spring replacement clutches left in stock, not even the canadian company I listed.

After some more searching, PontiacGP helped me find a guy on V8Buick.com who started producing his own. He setup production with a manufacturer and has small quantities produced at a time. He started doing this for people just looking to re-service their factory 8.5 units with S-springs. He charges the same price as the GM units (before they were discontinued), $199 + $13 shipping to Canada. He can be contacted thru [email protected] ; I exchanged a bunch of emails back-n-forth, he's a very nice helpful guy.

For someone just looking to re-service the slippy clutches in the factory configuration, this is definitely the cheapest option. You can pull the carrier, making sure you keep the shim-stacks in order, replace the spring and clutches, and throw everything back together in the same way.

My scenerio is a little different,
because I am also installing larger gears, the carrier needs a full re-setup. I'll need a complete overhaul kit and some setup tools. It's something I'm not so confident doing my own yet, and after getting quotes for parts and labour, the costs are looking similar for new Eaton carrier vs Factory Carrier w/ new Clutches.

It'll suit my racing better too, as I can order an Eaton 4 spring unit with parts more suited for racing (higher pressure springs and carbon fiber cltuches). I'm planning to order a carrier + rebuild kit for this summer, and work on it with someone who can go through it with me.

I've had to do alot of research to learn all this stuff, and I might be able to give you some pointers for your differential... But I've done much of my learning from guys like PontiacGP; you should post on here and he can give you some tips.

 

[khan0165]

Khan, I just rebuilt an Auburn today for a buddy who owns a shop. It was in a toyota truck and it has the S spring but the smaller rounded tabs on the clutch packs. Seems like Auburn switch the style of retention springs they use. My 67 Auburn has the 4 spring set up

yeah, I'm all confused with which years had what setup... seems like GM was all over the place around the late 70s and 80s
from what I can tell, the 8.5s produced in the late 70s had the more advanced 4 spring setups. But then GM decided to revert to the crappier S-spring setup through the later 80s.
And lets not forget the exploding Gov-locks

your 67 unit, are you talking about the 8.2 in your GP? I thought that had an Eaton unit? And I thought the Auburns weren't rebuildable? Maybe the Toyotas had rebuildable Auburns designed like the Eaton units?

 

[Fader]

Some good info. An 8.5 rebuild/add posi will be in my future so I appreciate the info & links.