12 Bolt Differential Question

[kenm78]

I have a 12 bolt rear end with what I believe is 3.42 gears. they seem to be ok but I was wanting posi or a locker. Who has done this and what is to expect. I have heard stay away from Motive Gears! But if I want to tackle this myself what should I look for or go to. I was thinking of lowering the gears to 3.55 or 3.73s. (Tired of one tire being bald and the other not)

[kenm78]

Has anyone ever swapped to a posi and was it worth it? What type of differance do you see. (never drove a vehicle with posi) ????

[green724x4]

I am looking into doing a diff locker swap on my truck. My buddy has a 96' k 1500 that has a Auburn Gear locker and he claims it works wonders. He used to have a eaton and he said that it would make a loud clunk everytime it would engane. This truck has seen it all from mud pits, snow banks, ect. and he hasn't had any problems. hope this was of some help.

[mark's68]

I used a mini spool with 3.73:1 richmond gears. Very easy to install. Turning sharp takes a little effort, but it hooks both tires.

[piecesparts]

I swapped out my son's 86 Chevy and my 91 chevy to Eaton posi units. The reason that I did, I wanted to get better traction and performance. My 84 GMC came with an Eaton 12 bolt posi with 3.42 gears and I changed it over to a 3.73 gear ratio for better driveability with my cruise control. My son's truck was a 3.08, 10 bolt open rear end and we installed a 12 bolt 3.73 posi geared rear end for the same reason. I definitely enjoy the posi units and in bad weather it makes a whole lot of difference on my daily driver. My posis are virtually trouble free and there is minimal noise on going around corners. Make sure you use the GM gear additive for a posi unit.

[crazy longhorn]

Generally, the Eaton limited slip is the best all around street friendly deal(not the cheapest tho). If you are going 3.73(you wont find a 3.55 for that one), you need to go 4 series on the carrier. 2.76-3.42 is a 3 series carrier.....3.73/4.10/4.56 is 4 series. best of fun, crazyL

[piecesparts]

It depends on who you talk to for what fits where. My 84 GMC has the 3 series smaller carrier, but I got a 3.73 gear set for it and replaced my 3.42 gears. A short talk to Randy's Ring and Pinion or Eaton directly; will give you insight on what fits your application. The difference is the thickness of the ring gear and it's placement on the carrier. I do agree the Eaton is the most amicable to install, becasue of the roadability, but the price is comparable to many other than an Auburn setup. My 84 has over 200,000 miles on the posi unit and it is still in good shape.

As for driving with a posi. On a dry highway, the noticeable difference is more in the acceleration and the fact that you don't spin your one tire and not the other. I also find that when it does that nasty stinky spring rain, where the streets are nothing more than an oil slick, my posi units get traction when everyone else is spinning at the stoplight. In snow and foul weather, I get around quite well with my posi unit.

NOW FOR THE BAD THING: If you are driving along and break traction on an icey road, then both wheels are spinning not just the one and if you stay in the pedal, the ride is more magnificent than if you had an open differential. BEEN THERE DONE THAT. However with the posi that little incident is not a normal occurence. I have had my 91's posi for 2 winters and it is great compared to the 3 winters that I had it without the posi. I drive a lot of miles every year, too.

[kenm78]

Thanks I was unsure about the weather issue and how a posi worked in it. How do I tell the differance between the series of carrier??? that is the first I have heard of this.

[kenm78]

Anouther ?? Can I change my current gears over to Posi I believe the are 3.42 or should I go up to 3.73???

[crazy longhorn]

To swap your current gears to posi, you need a 3 series case. The break I listed was for factory gears.....as piecesparts mentioned, you can get aftermarket "thick" gears, to run 3.73 or numerically lower gears on the 3 series carrrier, if you plan the gearswap AFTER you have installed the posi unit crazyL

[mnunn454]

I installed 3.73's and a new Eaton Posi in my BBC 69 a few years ago. In retrospect, I probably should have gone 3.42 to keep the highway revs down a little. Swapover was straightforward, but you will spend some extra time getting the pinion depth set properly.

As far as driveability, no noise at all out of mine, corners, straight, powerbraking, etc.

What I will say is that positrac workks better at maintaining traction, up to a point. Once the roads are slippery enough (or the foot is heavy enough) to break both tires loose, now the whole back end of the truck starts to swing around. If you aren't expecting it, you can end up in the ditch or worse. With one-wheel-peel, that seldom happens.

Finally, I hear that powertrax units are designed for the person who wants posi, but doesn't want to change the ratio or go through the hassle of setting up the R&P.

[Ackattack]

I put a Eaton Posi in my 72 GMC. First I had 3.08s (so its a series 3 carrier). When I swithced to the 700r4, I also switched to 3.73s and was able to keep the posi. I've taken it to the drag stip and cruising down the highway and I haven't had any issues with it. You just have to be smart about driving in the snow, just like any other time.

I also put one in my 68's 12bolt with 4.10 gears, but it will be awhile before its on the road.

[kenm78]

Thanks for all the replies been priceing some units and boy are they proud of them bad boys!!!!

[Ovidiu]

I didn't swap in a posi in my truck yet (other priorities so far, the resto is on hold), but on my camaro I put in a Torsen LSD (gear based).

I have researched it for the 12-bolt a bit and I will get (when time comes) a Detroit TruTrac (also gear based), I've heard plenty of horror stories about clutch-based LSDs.

[KFreddy]

Try these guys http://www.drivetrainspecialists.com/

No joke here these are the guys that built the rear end for project Copperhead, and their prices are right on the money. They put together a 12 bolt, 3.08, Eaton posi/ limited slip package for me that is backing a hard hitting ZZ454. Doing it once the correct way is the only way to go...

-Fred