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12 bolt truck rearend problem...?

2K views 4 replies 2 participants last post by  Bumpstick 
#1 ·
I was running a 3.73 geared 12 bolt truck rearend that was out of a 71 Chevy PU and was using a Powertrax performance locker (replaces the spider gears) this is not the reg Powertrax unit but a forged steel street or strip unit ($400+ new) that I got in a trade...

Anyhow I also had a 70 model rearend with a positrac (factory unit) w/ 4.10 gears and took the posi unit out and switched ring gears so I could use the factory posi unit with my 3.73 gear and eliminate the Powertrax unit...

The problem now is I think I have 2 different versions of carriers...? Because the shims for the 4.10 gear are about 3/16th inch thick and the ones for the peg legged 3.73 unit were only about 1/16 inch thick...? If I center the posi unit with the 3.73 gear on it (to match my pinion gear) then it has way to much slack or backlash... I can use the various shims to get proper backlash but it moves the carrier to the right by almost 5/16ths of an inch...? Can it be ran this way...? Or...? Should I get a spacer and re-space the ring gear and center the carrier or say screw it all and just get a new ring and pinion to match my carrier and from all this mumbo jumbo which carrier do I have (the posi one...)

thanks.... :)
 
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#2 ·
either you have a twelve bolt passenger rear and a truck rear or you need a ring gear spacer. I have put a twelve bolt pass center in a twelve bolt van but it was a lot of work and hard to come up with the correct shim combo.(I have about 25 lbs of chevy shims to select from)
 
#3 ·
Thanks for the reply Bob but these are both truck rearends for sure... The both have the panard rod castings and post... (did not come on any car rearends...)

My main question is should I get the spacer for the ring gear OR can I just use spacers to move the carrier over another 1/16th of an inch to correct the backlash issue...? Will it affect anything by off centering the carrier by about 3/16ths of an inch...? The axle play or movement is not a problem they move over 1/2 inch side to side... thoughts...?
 
#5 ·
Thanks again Bob... I found the part number for the spacer 902A from Mr Gasket should do the trick... Cost was only $39 and from all places (Autozone...?) was the cheapest and could get it the quickest...

I did talk with a buddy that has a shop that does nothing but rearends... He said I could use the spacer but also you can shim the carrier from one side to the other a full 1/4" if need be...? But it would be better to center the shims and use a ring gear spacer to get it closer... The problem is for sure a series 3 ring gear on a series 4 carrier... Thanks for your help... :) I`ll let you know how it comes out...
 
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