Hi I am trying to figure out what gear ratio my diff has, but I haven't been able to find the marking stamped in the axle tube. Now I've read you have to really scratch away the rust so ill work on it some more. But I have been able to find some markings on the inner side of the diff. housing but I don't know what they mean. 1235197,212, and then a couple of A's one an each side and then either a N or a Z. Is this the corporate 8.5 or has someone swapped it out? Any help is appreciated thanks.

 

The axle code will be stamped into the rear axle just beside the axle vent tube. Usually that means you have to be looking down at the axle from on top to see it. It will have a 3 lettters and 3 numbers. My axle code was SA O101. That translated to 2.73 gears for Olds axle assembled on the 101 day of 1972. I've since changed to a 3.42 so the axle code on the tube is really worthless (although I kept the OEM ring/pinion). Yours may have been altered before you bought the car. So I suggest you do the following to easily determine your gear ratio.

The numbers on the differential housing won't tell you the gear ratio. You can certainly take the diff apart to count ring and pinion teeth, but there's an easier way.

Lift both rear wheels off the ground. (necessary even if you don't have posi)
index (chalk) a mark on the drive shaft and outer wheel.
Have someone turn the wheel 10 full revolutions slowly while you count the revs of the drive shaft.
The number of revs you count divided by 10 will give you the drive gear ratio

 

Yes,that is an 8.5" 10-bolt.

 

That is 2 for the 8.5 10 bolt

pull the cover and spin the ring till you find the stamped ratio on the our
ter diameter (reference pictures)

jim
jd

 

I can turn my Head and move my Hand(that's really quite interesting) I'm getttttting Dizzy!

 

Allan, I don't mean to walk on this thread but I have been trying to identify my rear end ratio this way and keep coming up with approx 1.25/1 . Since we all know this isn't right any ideas why? I am making sure that there is no slippage.

 

Sampson, I posted my reply on your build thread.

 

Not that it really matters, but out of curiosity, might you know what plant "SA" was?

Can't seem to find the info and my axle was produced shortly after your was....

 

SA is the letter code that identified 2.73:1 gears on my 72 Cutlass. This information is contained in the 1972 CSM on page 0-5. SA is not a plant code.

 

Ahhh!
Thank you very much.
Still trying to get the hang of how all the different parts are marked...

 

I just figured out my gear ration Sunday.
Lift both tires off ground.
put car in neutral.
find a mark on the tire and turn twice around.
count how many times the drive shaft turn and thats the ratio.
Mine was 2 3/4= 2:78s

 

The 68 442 with 400 engine should have had a 4.33 gear ratio.

BTW - You will get absolutely best results by indexing both drive shaft and wheel, then turning the wheel 10 times. Divide the number of turns of the shaft by 10. Aside from counting the teeth on the ring/pinion this is a tried and tested way to confirm the actual gear ratio. Turning just once or 2x doesn't give that much accuracy. What if your axle is a 2.56? Turning the wheel 10 times means you should have counted the drive shaft turn either 25.6 times or almost 28 times. That's why it's more foolproof.

 

4.33, Really?
If thats what you come up with your method, I think I'll stick with the way I have done it for the last 30 years!
If a person can't see the drive shaft turn 2.5 or 2 3/4 turns then anything else on a car is too difficult to do for ones self.

 

According to the Oldsmobile Chassis Service Manual for 1968 442 with a 400 engine - yes really. Look it up if you don't believe me.

 

Just for interest sake, my 68 400G has a 3.07 rear end......

Ted

 

The ratio options for 68 442/W30 were 3:08,3:23,3:42,3:91,4:33,4:66,5:00
The Canadian cars could have 3:07,3:31,3:55,3:73,etc. because they were equipped with a 12-bolt Chevy.