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.
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 ,
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.
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 !!
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
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.
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.
This is an older thread, you may not receive a response, and could
be reviving an old thread. Please consider creating a new thread.
Chevy Tri Five Forum
A forum community dedicated to 1955, 1956, and 1957 Chevy owners and enthusiasts. Come join the discussion about restoration, engine swaps, reviews, performance, modifications, classifieds, troubleshooting, maintenance, and more! Open to all models including Belair, 210, 150, Sedans, and Nomads.