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3 series vs 2 series 8.5 GM 10 bolt

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3 series vs 2 series 8.5 GM 10 bolt

JoePentium JoePentium
User | Posts: 174 | Joined: 09/10
Posted: 11/16/11
05:16 PM

I purchased a 10 bolt/8.5/28 spline/ internal posi unit from a 77 Trans Am.
I found out after the fact that it was a "2" series, not the "3" series
I need this unit to be able to replace the original 3.08 open unit I currently have in my 73 Grand Am.
I read that I can purchase a spacer kit with correct length bolts and/or an aftermarket ring gear and pinion set. I planned on using either a 3.23 or 3.42 gear and placing this unit in to my original housing.
My 10 bolt housing has the squared casting at the bottom of the cover.
Can anyone offer any information about being be able to swap out the existing open unit and replacing it with this unit?  
"If programmers were architects, a single woodpecker could wreck civilization."

Opontee Opontee
Guru | Posts: 1422 | Joined: 07/09
Posted: 11/17/11
05:55 AM

From what I remember, the 3.08 is  a 2 series carrier. 3.23 and higher numerically (lower) is a 3 series. The easiest way to find out is is remove both and stand them up on one of the bearings next to each other. The spacer is about 3/16" thick, so if they are that far apart then you need the spacer. if they are closer then you don't. Don't forget to measure the backlash before you remove the old one so you can get the backlast set right, when you install the new one. Hope this helps.  
Anna Mae quit. Opontee

pepsi1 pepsi1
Enthusiast | Posts: 348 | Joined: 09/11
Posted: 11/17/11
12:20 PM

I ran into that same problem with a 79 Z-28 and I went from a 3:42 gear to a 4:10. I used a set of ZOOM gears and the ring gear was much thicker so I didn't need a spacer. Good Luck. Confused  Cool  

tuffnuff tuffnuff
Moderator | Posts: 2567 | Joined: 12/09
Posted: 11/17/11
12:56 PM

Ditto.,. in a 77 Trans Am, I went from 3.08 to 3.9's with Zoom gears, no spacer or problems.

When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

barneyformula barneyformula
Addict | Posts: 3306 | Joined: 01/07
Posted: 11/17/11
01:57 PM

My Formula has a set of 3.08's out of a 74 Camaro (I've had them a long time!) and they are bolted to the posi from a 77 Trans Am with 3.42 gears (actually the whole housing is from the 3.42 TA) and no spacer. I've never checked it out, but I'm told that the 2.73's in my original housing will also fit and that the 2.41's and 2.56's use a different carrier but I doubt this is true, i think the 2.73's would go on the carrier for the 2.41/2.56's. Some day I'll check it out for sure.
A little help... 'cause we don't all have to learn the hard way!

65pmdgto1 65pmdgto1
Enthusiast | Posts: 510 | Joined: 12/07
Posted: 11/17/11
07:44 PM

Those spacers make the a great start for a blown up rear end. Before the gear makers got with it and started making the thicker ring gears the spacers were common. If you have any intention on putting some Pontiac power through this rear end use the thick gear setup.  

JoePentium JoePentium
User | Posts: 174 | Joined: 09/10
Posted: 11/20/11
01:52 PM

Well I screwed up by purchasing a 2 series carrier posi unit.
Now I am going to either try and find a 3.42 gear that will fit a two series or sell this one when it gets here and purchase a three series unit, this is what I was trying to avoid.  
"If programmers were architects, a single woodpecker could wreck civilization."