Hey all,

So, I've done a bit more reading into rear end stuff, and I'm curious. My car is a '91 with a stock drum brake, non posi rear end. To my understanding, all 90+ rear ends are the better 28-spline 10 bolts? (as opposed to the 26). I'm wondering, because I originally had the idea of seeking out a 90-92 third gen disc, posi rear to put in my car, but they seem a bit hard to come by compared to earlier years, and asking prices can get a bit high.

I've noticed that FlynBye Performance sells a 1LE/LT-1 disc brake conversion kit for $85. If I had the idea of replacing my brake stuff anyway when I get around to making the car perform better, would I be better off just buying a disc-conversion kit and putting in a posi differential on my existing axle? It's starting to sound like it might be easier than hunting down a 1LE rear end, and I'd prefer to stick w/a 3rd gen rear, especially if mine is already 28 spline. Thanks for all/any advice!

 

If you find a 3rd gen posi rear end chances are that the posi (Auburn) will be close to the end of it's life. Then you will be replacing it anyway. If you upgrade your rear end then you will know what you have. I recommend the Eaton Posi or Duragrip, not Auburn. You are looking at about $450 for the Eaton or $360 for the Duragrip, plus bearings and shims to install it, so about $45 more. One thing that you must know is what gear ratio you have. If you have a 3.08 to 2.14 then you are going to be stuck with an Auburn, or maybe you can find a Torsen in a 2 series.

 

I'd just get a LS1 brake conversion for your axle & rebuild the center section with a new posi, if needed.

I have a '91 spare drum-braked 2.73 non-posi 10-bolt sitting here for that exact same purpose.

 

If you wish to do PBR's, you can use flynbye's brackets to get you started (the company sells lines (both hard and soft), parking brakes cables and combination valve). Should you choose the LS1 system (Ed also sells the above accessories for that system), check the following link for the axle-flange modification how-to:

https://www.thirdgen.org/ls1reardisc

JamesC

 

The '91 rear end probably already has the 4 bolt brake flange. Disc brakes might bolt on without modifications.

 

Though big has mentioned a drum four-bolt flange in the past (and he has a ton of experience), I've never seen one, so should that be the case here, please include a photo if possible. The car in the link above, BTW, is a 91 'Bird with the typical drum axle flange.

JamesC

 

So all 3rd gen drum brake rear ends had the 3 bolt brake flange? I know that the S truck 7.5 went to a 4 bolt flange in the early '90s, so I assumed that the F cars did too. I know the disc brake rear ends had the 4 bolt flange. I've used the 7.5 S truck drum brakes on some 12 bolt and 8.5 rear ends.

 

Yeah, I was starting to think this would be better anyway, because as you guys said, if I blow the bucks on an old posi-rear from a junkyard, it's probably already been beat to hell and needs stuff replaced anyway.

Big gear head: What's the deal on the gear ratios? I currently have 3.08's in mine, but I eventually want to switch to 3.42 or 3.73. Nothing higher (lower) than 3.73 though. I'm eventually planning either a T56 or TKO swap, but more than likely the T56.

 

If you have a 3.08 then it has a 2 series differential. The 3.23 to 4.10 rear ends use a 3 series differential. Eaton only makes a 3 series Posi for the 7.5 rear end, so if you use the 3.08 gears you can't use the Eaton Posi, or Duragrip. You would be stuck using an Auburn or Torsen. The Torsen woudn't be bad, but avoid the Auburn.

If you go ahead and change the gears to 3.42 then you can use the Eaton Posi or Duragrip.

 

I see. Well, what's the main difference between a 2-series and a 3-series carrier? is it just the types of gear ratios that they can accept, or is there more? I'll admit, I haven't actually taken a look at the thing to see if it is in fact 3.08, but from what I can find, all 305 TBI 5-speed cars came stock with 3.08 gears.

I was originally going to go with 3.42 gears, but multiple people on here have told me to go 3.73 if I swap in a T56, due to 6th gear being barely usable at (legal) highway speeds with 3.42's.

And out of curiosity, what makes Auburn so bad? is Eaton the best? Thanks again!

 

The only differencebetween a 2 series and a 3 series is the location of the ring gear flange. The 3 series has the flange closer to the pinion center line. Auburn wears much quicker than the Eaton Posi and it fills the rear end with powdered metal. It's also not rebuildable. The Duragrip is the same as the Eaton Posi only with a different clutch material.

 

Gotcha. But, just to clarify, if I'm planning to go w/a more aggressive ratio than 3.08, I have to find a 2-series differential w/the 3.42 or 3.73 gears? Or is the whole issue moot, and it's just that the better stuff like Eaton doesn't come in a 3.08? Obviously, I have a bit more to learn here on what actually goes into making the rear end work!

 

These are 12 bolt Eaton Posi cases. The one on the left is a 2 series, center is 3 series and on the right is the 4 series. Notice that the only difference between them is the location of the ring gear flange.

The 7.5 10 bolt uses a 2 series for 2.14 to 3.08 gears. It uses a 3 series for 3.23 to 4.56 gears. There is no 4 series for the 7.5 rear end. If you want to use your 3.08 gears then you must use a 2 series differential. There isn't a 2 series Eaton Posi or Duragrip, so you are limited to the Auburn or Torsen. If you change to the 3.42 gears then you can use the Eaton Posi or Duragrip.

There are thick gear sets available for the 7.5 rear end. This allows you to use 3 series gear ratios on a 2 series differential. The 7.5 is the only rear end that has thick gears for the 2 series.