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Discussion Starter · #1 ·
I just tryed to install a auburn posi in my stock 55 rear and found that it won't work with my 355 ratio, but will work with my 336 ratio, says it will fit 370 and down!!!! anyone had this problem and can shed any light on this?? I have a stock 55 rear and a stock 57 rear 3rd member and neither will except the 355 r&p.
 

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Discussion Starter · #3 ·
Yes they are both down in my opinion, but you can't move the posi unit over far enough to get the proper backlash!! I could use a spacer but i really don't want to go that route !! You would think they would tell you if you have to use a spacer ,
 

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There is some difference in the 55 and 56 gears, here is a link to
the 1957 positrac service manual and it says that the gears from 55-56 rears cant be used with the positrac unit unless they are 4.11 or 4.56 and will have to be replaced with 1957 and up gears
you also may need to trim .140 off the ends of the axles but that may or may not be needed wth the auburn unit.

Here is a link to the booklet see section 5

http://chevy.oldcarmanualproject.com/booklets/57top30/5730smindex.html

Regards, Robert
 

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TyphoonTX, I think you have hit on the reason. Good work.

Unless this is a race car deal with slicks, I wouldn't hesitate a bit to put a spacer on it. Just my opinion.
 

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Discussion Starter · #6 ·
Typhontx----Thanks for the reply and the link to the diff rebuild, the 336 gears work but the 355's won't, both 3rd members are series 3, 308's-336 will work, but a spacer is needed for 355-370's, i suppose the difference would be that the pinion is a larger od on the numericly lower ratios, thus no spacer is needed, hope this helps anyone else out before buying an aftermarket posi !!
 

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I think the 57 and later years gm and aftermarket 3.08 to 3.73 gears should work on the posi's is the way I read the information in the booklet. I think the first year for 3.36 gears was 1957 models when they went to the 14" tires

Regards, Robert
 

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My 57 has a 56 3rd member with 3.70's. When we put in a stock posi unit we had to use a spacer. I read somewhere (I can't remember where) where the 55 and 56 3rd members were built different thus the need for the spacer.

I think RickL stated somewhere that on stock posi's for 55 and 56's you need to take off an 1/8th of an inch off the axles, but on aftermarket ones you don't.
 

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You're right, it's not the gear ratio, it's the year of manufacture of the ring gear. 55-56 rings gears are going to need a spacer, and 57-up won't. Ratio won't matter.

This is because the ring gear on ALL 55-56 ring gears is thinner than the later ones.

Of course there's only 2 "3 series" ratios for 55-56 anyway - 3.70 and 3.55. 3.36 and 3.08 only came in later models, that is why they don't require a spacer.
 

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Discussion Starter · #11 ·
Thanks Rickl, you just cleared up a lot of questions for me, would any one know if the after market r&ps are thicker than the stock ones,? PS: there is no better source for information than this site!! Thanks everyone.
 

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Discussion Starter · #13 ·
Thanks Rick l, I just talked to tom's differentials and he said that the new ring & pinion sets will work without a spacer, as long as i use the 57 3rd member
 
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