Hi everyone, the last time I posted on these boards it was very helpful. I have run into a new area I am not very experienced in. I have a 1971 Chevy C10 Longbed Truck. I have rebuilt it and I am planning on converting to a posi rear end. I am having some problems on how I should go about this.

I don't know off the top of my head if its a 10 or 12 bolt, I believe it would be a twelve however, I'll find out tomorrow and post that for everyone.  If it's possible, I would like someone to give me direction of hwo to go about this.  Should i buy a posi differential and that's it? Or should i also buy a new ring & pinion as well ( I want to have 3.73 gear ratio, I am unsure what I have right now ).  Or is it cheaper to buy a new rear axle by the time you buy all the parts? I have a good mechanic friend who can install it, Just need tips on what I should do before I go to him with the parts! If someone could list a company with products and what I should pick up ( I prefer low - mid cost ), nothing over the top, I would greatly appreciate it.

Most of those trucks have pretty low gears, I believe the tallest is 3.43:1 and some 3/4 ton trucks have 4.10's. If you have a friend that can set it up right I wouldn't think you'd need to buy a whole new rear-end, if the wear on the gears is acceptable, that should be fine too, but remember that the truck 12 bolt needs a posi unit specifically for the Truck 12 bolt case, don't try to fit a car 12 bolt posi in it, it won't work.

http://nationaldrivetrain.com/shopsite_sc/store/html/page35.html

http://www.moserengineering.com/Pages/Differentials/eatonposi.html

I've owned many of the 67-72 trucks (11... I think) and they have had either 3.07, 3.73 or 4.11 gears.  If you get lucky and find that you already have the ratio you want, you can either find a good used posi unit or buy a new one (I've used auburn with good result). Then just remove the original diff., put the old ring gear on the new posi unit and shim it accordingly to acheive the proper backlash.

Your truck most likely has a GM unit with 8.875 inch ring gear, 12 bolt rear end which is not the same as a passenger car 8.875 inch 12 bolt. The truck is a much heavier duty unit. Parts between the pass 12 bolt and the truck unit are not interchangable.

You probably need to see what ratio you currently have, if you don't have the original paper work, jack up the rear end. Put a chalk mark on the input flange and a tire. Rotate the shaft counting revolutions on both the shaft and the tire. Divide the shaft rotations by the tire rotations and you have the gear ratio.

Chances are the guts of your current rear end are good. Unless you way over stress these things and run 'em dry, about the only wear item is the outer wheel bearings. Inner bearings and gears seldom wear out.

ARB, Auburn, Detroit Locker, Eaton, LockRight, NoSlip, Powertrax supply posi kits which range in price from about 300 to nearly a 1000 dollars.

Bogie

I disagree that the 8.875 12 bolt truck unit is a heavier duty unit.  Ring gear diameter and axle diameter and spline count are equivalent to the passenger car unit. But the truck pinion stem diameter is only 1.437 vs. 1.625 on the car unit. I say the car unit is stronger.  

Put a pass 12 bolt in a truck, put a working load in the back or hook up loaded trailer and see how long it lasts!

On the other hand put a truck 12 bolt in a passenger car and drive it 140 mph and see how long it lasts!

Bogie

Feel free to elaborate.  You're talking about two differential assemblies that are essentially identical in design.  Though as I stated before the pass. unit does have more beef in the pinion stem which tells me there is no reason it could'nt handle the load of the truck unit.  And the truck unit is no more Ill equipped to handle sustained high speed then it's passenger car counterpart.  

My experience in having done a few of these swaps over the years is two fold. First, I've found the pass 12 bolt to dislike having to work in a loaded truck, the problem being with the inner and outer axle bearings, I suspect this has to do with bending moments in the axle tubes or axles themselves. Second, is getting them aligned, the passenger and truck units like to work at different angles of drive shaft and in the case of coil sping suspension, different alignments of the trailing arms. Get these wrong and the rear end binds things up as it moves on the springs.

If your a conservative driver and can set the suspension up properly, either can probably work just as well, but few people use a 12 bolt because they're a conservative driver.

Hi, I have a 78 k5 blazer w/ a 12 bolt. I decided to put a posi in it. 1st step: is identify what rear end you have. 2nd step: research what kind of posi you want to use. I went w/ a eaton posi that i found on craigslist, used w/ 373 gears and in exel condition. The 64-81 truck years are what you want to look for. Im doing this my self. Not recomended if you dont have the proper tools. Buy a master install kit. along w/ your posi carrier, and new gears, and send it to your mechanic. you can buy the gears and install kit off of ebay at a good price and have them shipped to your front door. Make sure your parts are for a truck and not a car. The car uses a larger dia pinion, and will not work in the truck. Hope this helps.