GM 10 Bolt Rear End Questions

Recently I enabled my buddy into starting a 2015 Challenge build with a Buick Roadmaster wagon. Buick Build Thread

He and his children are currently in the part gathering stage of the build and one of the first things on the list is a better gear for the rear end. The wagon came with a 2.56 rear ratio which is less then ideal for drag racing or autocross. My understanding is this ratio uses a "series 2" carrier and is not compatible with a higher ratio gear sets.

We were thinking of going with a 3.43 rear ratio and posi, which means using a "series 3" carrier. My question for the brain trust is will a "series 3" carrier fit in the stock Buick rear housing or do we need to source a whole new rear axle assembly. The current rear end is a 10 Bolt 8.5 with 30 spline axles. Will the axles work with the new gear set if it fits or will we need to change them out as well.

I am looking at something like this on ebay...

They seem to trade in the $250 range for a used one. Is this all I will need or should I be looking for something different.

http://www.ebay.com/itm/gm-posi-lsd-locker-10-bolt-8-5-342-gears-gmc-chevrolet-1-2-ton-/381355549657?hash=item58ca8fe3d9&vxp=mtr

http://www.ebay.com/itm/TIGHT-8-5-Posi-10-bolt-30-Spline-3-42-gears-gov-lok-eaton-locker-GMC-chevy-lsd-/261996686318?hash=item3d00382bee&vxp=mtr

The rear axle housing is a corporate 8.5", not specific to Buick, so a 3-series carrier for an 8.5" 10-bolt with 30-spline axles will work. Early cars had 28-spline axles so watch out for those.

Both of the diffs you linked from ebay are the gov-lock style, which are fairly weak, especially for performance use. I would get an Eaton or Yukon clutch style if possible.

Is this one a better choice?

http://www.ebay.com/itm/GM-8-5-Posi-10-bolt-30-Spline-Eaton-3-series-/161793844791?hash=item25abaa2637&vxp=mtr

Cop cars usually had 3.08 or 3.43. See if you can find an Impala SS guy that upgraded.

The "series" has to do with the location of the ring gear mounting flange and nothing else.

If you think about it, the ring gear is always the same diameter, so for the ratio to be different, the pinion gear has to change diameter. The smaller the pinion, the further over to the right the gear mesh is. In order to prevent having 5" thick ring gears on the shortest ratios, they had different "series" differentials. 2-series diffs had the ring gear flange the furthest over to the left, 4-series diffs had it furthest over to the right.

This is why you can get, say, 4.56 gears for 3-series differentials (they will be really, really thick though) but you can't go the other way.

Incidentally this is also why shorter gears tend to be weaker. The pinion ends up getting really small.

Knurled wrote: The "series" has to do with the location of the ring gear mounting flange and nothing else. If you think about it, the ring gear is always the same diameter, so for the ratio to be different, the pinion gear has to change diameter. The smaller the pinion, the further over to the right the gear mesh is. In order to prevent having 5" thick ring gears on the shortest ratios, they had different "series" differentials. 2-series diffs had the ring gear flange the furthest over to the left, 4-series diffs had it furthest over to the right. This is why you can get, say, 4.56 gears for 3-series differentials (they will be really, *really* thick though) but you can't go the other way. Incidentally this is also why shorter gears tend to be weaker. The pinion ends up getting really small.

Thats good to know. Well put.

Well put Knurled. The 8.5 has 2 and 3 series diffs, 12-bolts had 2, 3 and 4 series. There is some good info on 8.5s here: http://www.superchevy.com/how-to/48518-posi-10-bolt-upgrade-install/

This is a picture I took of 3 different gears for a 8.5. Left to right is 2.56 (2 series), 3.73 and 4.30 ratios (3 series). Easy to see the differences.

I've got a 10 bolt with 3.23s and a torsen out of an automatic ls1 4th gen Camaro I would be willing to part with. Not sure if the width would work...

In reply to Furious_E:

Is it an 8.5? If it is a 30 spline it should work, what would you want for it?

Bahh, The Google says its 7.625.

The Roadmaster I believe shares the same 5x5 wheel bolt pattern as several full size Chevy/GMC trucks (that came with 12 bolt rears, like my 79 GMC). I wonder what the width camparison is - how much welding/fab/cutting etc to fit a GM 12 bolt in?

I have a street rod/rat rod/whatever project in my head, that I'm thinking "would a 14 bolt 3.73 locker from a 99 up GM Truck work? 8 bolt wheels

Appleseed wrote: Cop cars usually had 3.08 or 3.43. See if you can find an Impala SS guy that upgraded.

93 and earlier TBI cop cars had 3.42 gears and usually a posi but had 11" drum brakes... 94-96 LT1 cars had 3.08 gears and usually not a posi, but they also had 12" disc brakes.. the 94-96 cars could also have the puny 7.5 rear with 3.23 gears if it has the 4.3 liter L99 "baby LT1" engine, so keep an eye out for that and avoid.. the 3.42 gears really wake up a small block without killing fuel economy. so instead of screwing around with gear and differential swaps, just find a whole rear end out of a 93 and earlier 9C1 and bolt the whole thing in.

Here is a link to some older data

http://buickforums.com/forums/threads/15788-Roadie-rear-end

and some more

http://www.impalaforums.com/buick-roadmaster/279375-1995-roadmaster-lt1-rear-end-swap-posi-installation.html

wagon and sedan have different rear axle widths and the lower control arm points are wider on wagon.

these cars are 19-25 years old now, it's not as easy to run out and snag a 3.23 posi tow package wagon axle as it used to be.

In reply to TIGMOTORSPORTS:

A 14 bolt would be way to heavy and overkill for this application. Also the drum brakes on most 14 bolts weight something in the range of 65lbs per side.

I wonder about the Trailblazer SS's 14-bolt, though.

The standard Trailblazer has a magnesium 10-bolt. Pretty sure it's an 8.5 truck rear and not a 7.625 passcar rear.

There are 14 bolts that have disc brakes from the factory

I do have an 8.5 30 spline 3 series pickup locking rear center section on the shelf that'll work with some used gears that I'd let go cheap for a fellow challenger. There really is zero need to upgrade from the 8.5. No wagons got the lighter 7.5/7.625 rear

patgizz wrote: I do have an 8.5 30 spline 3 series pickup locking rear center section on the shelf that'll work with some used gears that I'd let go cheap for a fellow challenger. There really is zero need to upgrade from the 8.5. No wagons got the lighter 7.5/7.625 rear

Perfect send me a PM and we can work out the details.

At one time, I had a 69 Buick Electra with a 430 that weighed about 4600 lbs with no driver - ran 14.70-14.90 at 93-95 mph with used McCreary circle track DOT tires. Factory 10 bolt didn't have a problem with the torque/weight combo.

I would recommend a quality synthetic lube to help take the heat.