Hello all,

I've been a member for a while, just reading and lurking since I had to sell my last 70 when I was laid off. I am purchasing another and one of the things I want to change is the rear gear. Lots of RPM at highway speeds. Anyway, once I have the car, I can check the axle code, but I am not certain that the internals will match as this car has been rebuilt. The current and 4th owner does not know what gear is in the car. He suspects 4:11s. I will check gear ratio once I have the car by doing the "count the rotation thing."

However, after reading many posts I notice comments about 3 & 4 carriers. I assume this is a type of ring and pinion carrier. Can someone give me some background on these two? What is the difference between the two and how to tell?

Many thanks!
E.

The GM rear ends have different series differentials, or carriers for different gear ratios. For example, the 12 bolt uses a 2 series for 2.73 gears, a 3 series for 3.07 to 3.73 gears and a 4 series for 3.90 to 6.XX gears. There are some special gears made to fit other series differentails, such as the thick 4.10, 4.56 and 4.88 gears that fit the 3 series, and the thin 3.73 and 3.90 that are made to fit the 4 series. The difference in the differential is the location of the ring gear flange. A 2.73 gear has a much larger pinion head than a 4.88 gear, so the 2.73 ring gear must be moved farther away from the pinion center line. This is why there are different series differentails. Ford didn't do this. They just adjusted the thickness of the ring gear to make up the difference.

Thanks for explanation. I will have to see what is in the car once I have it. I read in a different post that the carrier number is stamped on the inside away from the ring gear. Correct? Does the last digit designate which series it is? Are the carriers readily available?

Thanks,
E.

The series number is not on it, but most have a casting number that will identify it. Eaton makes new limited slip differetnals for most of the popular GM rear ends in 3 and 4 series. Auburn and a few others also make them, but I prefer the Eaton.