It is a pain to try and get identification info for an Olds differential. Anybody have a better way to tell if this is a Type C or a Chevy 12 bolt? It has the crease in the cover and 12 bolts in the cover but the numbers are stamped on the front of the passenger side tube. KDC 1232. The axle has been molested at some time in the past judging by the state of the brake lines, so it may or may not be the original to the car. Any help is appreciated.

 

Type C IS a Chevy 12 bolt and yes, that is what you have.

 

Well, that clears it up. Thanks Joe. I didn't find anything in my searches to connect the two but maybe I should have realized the "C" stood for Chevy. D'oh!

 

Yeah, it's "Type C" as opposed to "Type O".

 

KD means ratio was originally 2.73
C means assembly plant was Buffalo NY

 

Thanks!

 

So, now that I know more about what I have, I am anticipating changing the gear ratio. There is a local guy selling two different ratio gear sets for a 12 bolt, 3:42 and 3:90. I have a 427 CI Olds small block, a TH200-4r in a 67 Cutlass convertible. Any opinions? I will be driving the tires off this thing as much as I can. What kind of performance difference will the two ratios have? Mileage is irrelevant at this point, I am not building an economy car! Thanks in advance for opinions and input!

 

I would get the 3.90's.

 

I run 331 gear ratio in my 12bolt chev axles
I had 373 and that was just too deep when I hit the autobahn LoL traffic around here 80MPH or stop protest on the highway

of your picks I would vote with 342

 

Right Now JEGS has $100 off on 12bolt posi units and FREE SHIPPING


Yukon Duragrip YDGGM12P3301
This unit is for 3 series gears and 30 spline axles

 

I agree, that should give you awesome acceleration and good highway gearing.

2.74 first gear x 3.90 rear gear = 10.686 overall effective gear reduction. This would be the equivalent of a TH400 with a 4.30 rear gear.

Overdrive would give 0.67 x 3.90 = 2.61 equivalent gear ratio.

 

I like the end result in overdrive, but that starting ratio is kinda low. This is not a high winding engine so I would be shifting into second at about, what, 20 mph. Mark recommended a 3:42 and I was wondering what the difference would be. Ultimately, I think he is right. With the stump-puller torque and 5500 RPM max, the slightly higher gear might work best. It might get me to, say, 30 MPH before 2nd gear.

 

I would not worry about shifting out of first at 20 MPH as the 1-2 shift will happen at the same engine RPM as it would with a 3.42 gear or even a 2.56 gear. A friend of mine put a TH700-R4 (3.07 first gear ratio) and a 4.56 rear gear in his car and loves it.