wagsk2000

how do I go about switching my non posi 12 bolt to a posi, and how do I determine what gearing I have. My car is a 70 S with a 455 and 4 speed.

2blu442

Welcome to the site! If nobody has messed with the rear end there will be a two letter code identifying the gear ratio from the factory. It will be located on the passengerside tube, typically on the back but sometimes on the bottom, top or front and it might be upside down as in my pictures. Attached is a couple photos of this code. If you find yours and post it we'll tell you what the factory ratio was. To upgrade to a posi you'll need to replace the carrier with the correct one. If its the factory Type O rear end there's not a lot of carriers out there. A member named monzaz has a business that deals with rear axles. Here's an old thread about carriers he sells for these rear ends. John
https://classicoldsmobile.com/forums...osi-units.html

DSCN4492.jpg

DSCN4490.jpg

wagsk2000

thanks for the reply, on pass side tube R C and -0508K- on diff case 3959C38 the 9 could be an 8 also im not sure

Cman7713

http://www.442.com/oldsfaq/ofdif.htm

This is an amazing article with everything written out as clearly as possible. It helped me when trying to learn about differentials and gearing etc. I'm probably wrong but in the list provided in that article it shows RC code as a 3.08 open rear.

wagsk2000

Thank you, that article was exactly what I needed thanks for replying so fast

Octania

This part is bullshed:
"Determining Gear Ratio...TURN THE DRIVESHAFT. Count the number of turns of the driveshaft it takes to turn one wheel one time. If you turn a wheel, instead of turning the driveshaft, you may only turn the other wheel (not the driveshaft)" - typically this occurs when the differential is simple gears, not a positraction. Limited Slip, or any of the 100 other marketing names.

See post #21 of
https://classicoldsmobile.com/forums...tml#post840694

Specifically:
What you are shooting for, really, is one revolution of the RING GEAR and CARRIER. This requires TWO COMPLETE WHEEL TURNS- whether you get that by turn two wheels once... or one wheel twice... or 1.5 & 0.5 or any other combination, is up to you. Whatever works for you that day.

and:
If you have not seen the differential explained in the 1940's GM video on youtube, go see it now.

I believe it is this 2.5 million view thing:

joe_padavano

Just to be sure you get the correct parts, your axle isn't a "12 bolt". It's a Type O. The number of bolts on the cover is not the important number (despite what many people think), it's the bolts on the ring gear. Your axle only has ten bolts holding an 8.5" ring gear. Type O parts can be hard to come by, as the posi unit is unique to that axle.