It turns out my 1970 Chevelle SS396 L34 with TH400 has a 3.31 "pegleg" nonposi 12 bolt rear end (original). While I am happy with the 3.31 Gears, I'd much rather have a Posi rear end for the once-in-a-blue moon trip to the strip, and for safety on the road. How mechanically difficult and expensive is it to convert to a posi rear end? Can I simply change the gear set in my existing 12 bolt, or is it more complicated than that? Perhaps something from Auburn or another aftermarket supplier will bolt in? Can I use my axle or do I need to swap it out?

Thanks in advance for your help.


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Morris "moepoe" Porter

Aces 2674
'70 L34 Chevelle SS396

I had my 12-bolt done recently. Thought my posi was burnt and went in to have the unit snugged for $50. Turned out after we opened it up it was a peg. Let the C-clip loose and the bearings had chewed through the nice machined surface of the axles.

Making a long story short, New axles, studs, eaton posi, yoke, bearings, seals, gear 3.73, and related shop supplies and labor, $1400.

I do not know from first hand, but have heard bad stories about Auburn posi units, including the shop that sold me the Eaton.



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JohnC
Team Chevelle Gold #223

Radd, and everyone else,
DON'T THROW AWAY THOSE AXLES THAT ARE PITTED FROM A BAD AXLE BEARING! There is a replacement bearing on the market for that purpose, commonly called a "repair bearing", every parts house sells them, they are very common. If you can get a discount at a parts store like NAPA, they will run $19-25ea. It is a one piece bearing/seal. The bearing rollers are offset to ride on a fresh surface of the axle (be sure and clean the fresh surface real good, maybe with steel wool). I don't know about serious, hardcore racing, but for what these cars are being used for by most people nowadays, this may be the last axle bearing your car ever needs. After you remove the regular bearing and seal, the repair bearing will protrude out of the axle housing end about 1/4in after installation. Don't be afraid to drive it in, it's a tight fit.

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I had the same setup on my '70 SS396 until recently. Just could not get traction at the dragstrip (ET's 15 - 18 secs). Recently had the Auburn unit installed and upped my gears to 3.73. Didn't cost much (around $200) and it certainly fixed my traction problems. ET's now mid-13's. Definitely worth it.

Just wondering, you mentioned you wanted a posi for increased safety on the road? I'm not an expert but I know that if both your tires are spinning you have a good chance of doing a 360 finished off with a barrel role in the ditch. When one wheel isn't spinning it keeps its traction and keeps you in a straight line. I know a few guys who spin out in parking lots (winter) backing up and there a## end slides into the car beside them, that's caused 100% by the posi rear end. Just thought i'd throw that info at you, may not apply if you don't have winter.

Chris

Thanks Chris, for the clarification on safety. We have winter but my Chevelle never sees it. I was thinking of safety as in less fishtailing and better traction on a standing start. And less fishtailing when the gears chirp. Oh well, I guess I just like the idea for the strip, and I'd rather cook both back tires when I get childish and smoke 'em!!!! You have to make an ass out of yourself once in a while when you're carrying that kind of power!!!

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Morris "moepoe" Porter

Aces 2674
'70 L34 Chevelle SS396

An Auburn posi unit is as good as any other posi, but I was told by the shop that installed one in my ElCamino that the unit is unrebuildable. This means that if or when I break it, a new unit will be needed. I like the old GM posi's because you can still buy parts to rebuild them, and they can take a joke if you know what I mean.DTS did two 12 bolts for me and their prices are not out of line.

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Team Chevelle Gold #33
ACES Member #02913
Chevelle Club of Michigan

The Auburn is a pretty good posi, but as mentioned, it is not rebuildable. We usually ask the owner before we install a posi if they ever want to have it rebuilt, or if they are hard on the car. We recommend that they decide whether to go for the Auburn, or and Eaton. The Eaton is rebuildable, and is the same as the stock posi units. To convert to a posi, and use your gears, you may be looking at around $700-800 to have it done properly. Here is a breakdown:

Posi unit - $400-450
bearing kit - $130
shim kit - $45
labor - $185

To have new gears done at the same time will add about $3-400 to the price.

I would not recommend doing this job yourself due to the precision involved in setting up a rearend, and the cost of the parts involved. Take it to a professional driveline shop and have it done. Ask them if you can buy the parts yourself and have them install them. We do this at our shop, with no problem, and it is usually less expensive if you buy the parts yourself.

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70 & 1/2 RS Camaro
SB406 & M20 4spd
70 Malibu
SB350 & TH400