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Discussion Starter · #1 ·
I never thought I'd break a 12 bolt positrac with a small block. This is just my test engine so I can go through everything and find out where I'm at before I install the 496. I already went through (beefed up) the entire driveline BEFORE the rearend and I thought the 12 bolt would be fine. Today I made a few launches, and on the 2nd one I noticed the nice even front-end lift wasn't even anymore. It lifted great on the left side and the passenger side seemed to dig a hole in the road. I jacked up the rear only to discover that my once posi that turned the same way on both sides now turns opposite directions. I already plan on installing 3:73's, but what else should I plan on changing now?
 

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Sounds like worn clutch plates.
If it's an Eaton ,you can rebuild it ,if it's an Auburn ,it's a paper weight.
 

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Discussion Starter · #3 ·
Sounds like worn clutch plates.
If it's an Eaton ,you can rebuild it ,if it's an Auburn ,it's a paper weight.
How can I tell which it is? If I give you the numbers can you tell me? This wouldn't be a spider gear issue? The car still goes fine with no noise, so hopefully you're correct.
 

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Yeah, Eaton clutches do wear/burn out.
I "abused" up three sets, including a set of carbon fiber ones w/800 lb springs, on my 'old '70 within two years.
Got tired of that and finally went to a spool...
 

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No ,that's the casting number of the housing ,need the # off the carrier.
Yup ,gotta pull the cover to get to it.
 

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Discussion Starter · #8 ·
No ,that's the casting number of the housing ,need the # off the carrier.
Yup ,gotta pull the cover to get to it.
Thanks, Mike. I put it up when I get a chance.
 

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Here's the 12 bolt Eaton posi carrier numbers
ED32088 =2.73:1 & down
30140PM1 =3.08:1 to 3.73:1
EDB30174 =3.90:1 & up
 

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Discussion Starter · #10 ·
Cool, Thanks Mike. If it is an Auburn is it not rebuildable? Can I use the same housing to upgrade?
 

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Discussion Starter · #12 ·
Housing's fine.
Auburn's arn't rebuildable.
If it stock oem , it'll be an Eaton.
It came out of a 71 Monte. What will this one be?
 

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I'd say that it's probably a after market Auburn. They don't last long in drag racing. If it's an Auburn take it out and toss it in the scrap metal pile and get a new Eaton. You will have to clean all of the powdered metal out of the housing and axle tubes before rebuilding it.

If you do have the Eaton then you can rebuild it for drag racing. I recommend the 18 or 22 disc steell clutches with 400 or 800 pound preload springs. The parts you choose will depend on what you intend to do with the car. Shim the clutches for the correct preload.

If you have an Auburn and you replace it with a new Eaton then I recommend that you take the carbon fiber clutch plates out and replace them with a steel clutch set.
 

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Discussion Starter · #16 ·
Thanks for the input guys.
 

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I'd say that it's probably a after market Auburn. They don't last long in drag racing. If it's an Auburn take it out and toss it in the scrap metal pile and get a new Eaton. You will have to clean all of the powdered metal out of the housing and axle tubes before rebuilding it.

If you do have the Eaton then you can rebuild it for drag racing. I recommend the 18 or 22 disc steell clutches with 400 or 800 pound preload springs. The parts you choose will depend on what you intend to do with the car. Shim the clutches for the correct preload.

If you have an Auburn and you replace it with a new Eaton then I recommend that you take the carbon fiber clutch plates out and replace them with a steel clutch set.
Are you recommending steel clutches only for drag racing? What about street/strip/road course applications, will the carbon fiber clutches be okay for that?

Does installing seriously heavy springs cause the axle to cackle a bit on slow moving tight turns?
 

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The steel clutches will hold up to burnouts better than the others. The heavier springs also help in this area. Stronger springs will increase the popping in tight turns. It's not a good idea to put excess oil additive in the rear end because this just helps the clutches slip easier, which is not what you want for drag racing. For road racing and auto cross I recommend the Truetrac. For street use the carbon fiber clutches are fine.
 

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The steel clutches will hold up to burnouts better than the others. The heavier springs also help in this area. Stronger springs will increase the popping in tight turns. It's not a good idea to put excess oil additive in the rear end because this just helps the clutches slip easier, which is not what you want for drag racing. For road racing and auto cross I recommend the Truetrac. For street use the carbon fiber clutches are fine.
Great, thanks for the info. The wagon (plans for street, little strip, little road course) has a new Eaton posi with carbon fiber. The coupe has a standard old Eaton posi but for some reason years ago I put heavier than normal springs in it and it cackles pretty bad when pulling into the driveway and such. It's got a bottle of the GM Posi Lube in it, but it still cackles. Seems fine on the street in normal driving. I should probably think about replacing the springs with standard springs, it just doesn't need those heavy springs (full bore restoration/show car).
 

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The stronger springs will cause the clutches to wear quicker. The new Eaton with the carbon fiber clutches comes with 400 pound springs. This is probably because it only has 14 clutch discs instead of 18 and needs a little more pressure on the plates to help it lock up. The 200 pound springs are plenty for a street car with the 18 disc steel clutches. The clutches should always be preloaded correctly. This will help them lock up and also help prevent the pinion and axle gears from breaking.
 
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