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Discussion Starter · #1 ·
I will be undertaking putting in a posi differential in my Grand Marquis. I'm not doing anything fancy, just want to swap out the factory peg leg for a factory posi (keeping the original ring and pinion). The procedure looks straight forward enough, but I have a question about the backlash. Since the differential only will change, do I need to check the backlash and contact pattern? Both units are factory parts - could I get away with just putting back the shims that were there?
 

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Could leave the pinion alone, the depth and contact should remain the same, but the carrier side shims might change. Just have to do the install and check for lash, new carrier should come with a shim kit I'd think.
 

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Discussion Starter · #3 · (Edited)
Yea, I wasn't planning on touching the pinion, it's the side shims I was thinking about. I guess I should just be diligent and check it.

What sucks is not knowing if I'll need more shims. If I buy some and don't need them, I'll be out $$$$. If I don't buy them and need them, the car could be down for some time while I wait for the shims. :(
 

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If anything you made need to move the shims from side to side to get it right.

Make sure you measure backlash before disassembly and set it up as close as possible to the same.
 

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Measure the thickness of the new carrier vs. the old.
Since you aren't changing the ring gear that thickness will be the same.
The total thickness of the shim pack and carrier, and the opposite shim pack
will need to equal the same number. So, add to one, subtract from the other.
Change the shims side to side to match.
If there is no difference, you should be fine.
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Run a flat file across the face of the carrier (diff) to make sure there are no burrs at the bolt holes.
Follow the manufacturers torque specs for the gear.
My advice is you freeze the carrier and put the ring gear in the oven.
Heat the gear to 200*, it will slip on with no problem and you won't have to worry about damage when you torque it down.
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don't forget the friction modifier with the new gear oil.
 
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