I have a 64' f-85 with a 2.78 ratio in the rear end. I looked around for a posi unit that would fit this ratio and couldn't find one. I found auburn gear units that only fit 2.93 and up, nothing lower.

1. Does someone know where I could find a unit that fits 2.78?

2. Could someone explain what restricts me from using a 2.93 unit in my 2.78 rear end. Or, can I modify a 2.93 to work with my 2.78?

Thanks.

 

1.) Only Factory Buick & Olds posi unit from 1964-1967 can be used... Pontiac rear from 1964-1972 can be used (2.56 and 2.78 ratios use the same carrier)
NO AFTERMARKET 2 series posi units

2.) The carrier ring gear flange is to far to the passenger side to mount the ring and pinion with out the posi unit being too far to the driver side and then the axles will NOT engage the side gear correctly on the passenger side.

Info

2.56-2.78 2 series
2.93-3.23 3 series
3.36-4.33 4 series

Reference picture to see what makes the carriers 2 3 4 different

Jim
JD

 

I will check the used posi rack and see what we have. I think we only have 2.93-3.23 factory. Jim

 

Thank you very much for the help, is the two series the one on the left with the red springs?

Thanks again.