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Old 12-08-2004, 09:02:00 PM   #1
runnin350
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change to posi

So how easy(or hard) is it really to change out a unit and put a posi in? I have heard really difficult and piece of cake. Whats the deal?
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Old 12-08-2004, 09:32:00 PM   #2
big gear head
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I'm assuming that you have a 8.5 10 bolt rear end. If you have something else then some of this will not be correct. Remove the cross shaft and c clips and remove the axles. Before you remove anything else from inside the housing you must use a dial indicator to check the backlash on the ring gear. If you don't know what the backlash is then you will not know what to set it to when you install the posi. To check the backlash you must wrap something around the yoke and housing to hold the pinion perfectly still. If it moves ANY you will get an inaccurate reading. Set the indicator up so that it measures rotation of the ring gear. What you will be measuring is the space between the ring gear teeth and the pinion teeth. This is the backlash. This should be around .010 to .012 inch if the gears are pretty good. Now that you know what the backlash is you can mark the right bearing cap and remove the caps. Do not mark the left cap with a punch or chisel. This will cause a weak place in the cap and it could cause it to mreak. There is no need to mark both caps, and the left cap carried more load than the right cap, so only mark the right cap. You must mark the cap so that you know how they came off and you can put them back exactly the way they were. Now pull the differential out of the housing. There is no reason to mark the bearing races and shims because you shouldn't be using them again anyway. Remove the ring gear from the differential and install it on the new posi. Remember that these bolts have left hand threads. Torque the ring gear bolts to 65 foot pounds and use some Lock Tight on the threads. You should have new Timken bearings installed on the posi. Be sure that the bearings are seated completely against the case. You will need a good shim set to get the backlash set correctly. Install the posi with the bearing races and slip about .225 shim into the left side. Be sure that this doesn't jam the ring gear into the pinion. Now start adding shim to the right side until it is tight. When you get the shims tight install the bearing caps and check the backlash. There is no need to torque the cap bolts at this time, just get them snug. Make adjustments to the shims until the backlash is correct. When you have the backlash correct then you should add about .003 to .005 to each side until the shims are very tight and the differential can not be pulled out by hand. Check the backlash again to be sure it didn't change. Now torque the cap bolts to 60 foot pounds and finish assembling the rear end. If you use a 80W90 mineral based oil you will need to use a bottle of GM additive 1052358 in the oil.

Checking the backlash with a dial indicator.

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Last edited by big gear head : 01-19-2012 at 02:04:35 PM.
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Old 12-10-2004, 01:19:00 AM   #3
runnin350
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So as long as I take my time and check everything over it shouldn't be too bad, right?
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Old 12-10-2004, 06:52:00 AM   #4
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Great explanation, Big Gear Head!

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I thought this was supposed to be fun?!
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Old 12-10-2004, 09:16:00 AM   #5
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BGH certainly knows his shizznit!
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Old 12-10-2004, 12:48:00 PM   #6
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Why not just install a Powertrax?

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Old 12-10-2004, 01:47:00 PM   #7
classicz28
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You should right a book..... You really know how to explain things without being too techical... Thanks for the great info..
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Old 12-10-2004, 08:53:00 PM   #8
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BGH is the MAN. One thing you might consider, is a LOCKRIGHT setup. If you have an "open" differential, then for about $200 you could install one of these in your existing diff, and WHALLAHH!! POSI!! Did this myself, and it was pretty straight-forward.
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Old 12-11-2004, 12:49:00 AM   #9
runnin350
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Hmm.. this lockright sounds interesting. No problems? And it wasn't too hard? I'll look into that...Oh, and I found out the rear end is from a 75 Nova. Surprised me just a tad!
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Old 12-11-2004, 01:13:00 AM   #10
runnin350
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Auburn makes a product similar the that and its cheaper. Do you get what you pay for in this sense or is it ok?
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Old 12-11-2004, 10:23:00 AM   #11
big gear head
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If you are going to get a posi you might as well get the best one. The Eaton is better in my opinion. The Auburn has a few things that I don't like. The PowerTrax is easy to install, but doesn't operate as smooth as the Eaton. There are several options for the 8.5 10 bolt, such as the Detroit Locker, ELocker and some others. For a street/strip car the Eaton is probably the best for the money.

I'm trying to write a book, just can't seem to find the time.

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[This message has been edited by big gear head (edited December 11, 2004).]
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Old 12-11-2004, 02:33:00 PM   #12
runnin350
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Ok Im getting ready to order and am confirming the grear ratio. It is supposed to have 3.08. With one turn of the driveshaft the tire makes less than 1/2 rotation. On full tire rotation is just under 1 1/2 driveshaft rotations. Am I doing this wrong, cause Im not even getting close.
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Old 12-11-2004, 02:51:00 PM   #13
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Another vote for the powertrax lockright.

I installed one in my 88 camaro and love it.

The install was fast, and easy. No probs with the unit at all.

I thought the "clicking/clunking/banging" that everybody kept talking about was gonna be bad, but it's almost as quite as an open rear.

If you have a performance exhaust system, the only time you'll ever hear anything is at for slow speeds (5 MPH or less) turning into a parking space.

Turning tight turns (90*) out of a road at a stop sign the wheels will 'hop' alittle

And if you drop the throttle in the middle of a curve the rear of the car will 'wiggle' a twitch.

Wheel spin was cut in half if not 60% less than the open rear
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Old 12-13-2004, 02:13:00 PM   #14
big gear head
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Leave one wheel on the ground. Mark the drive shaft and the wheel that is off of the ground. Turn the wheel exactly 2 full turns and count the number of turns that the drive shaft makes. This will be the gear ratio. This is for a rear end with a standard differential. A posi requires a little different method.

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Old 12-13-2004, 03:47:00 PM   #15
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I re-used my original shims and the backlash came out right on the button where it was before. I don't know if I got lucky or not, but it is always worth a try before you spend the money on an un-necessary shim pack like I did.

:edit:

By the way, if you do get an Eaton, run the Ford friction modifier. It runs way smoother than the Chevy modifier does.

[This message has been edited by 73454 (edited December 13, 2004).]
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