swapping rear in a 69

[Greg 32781]

I have a 300hp 350 small block automatic, serial # 194679S708599 that someone had installed a cam in before I bought the car, it had a 3.08 rear gear from the factory. The code on the original rear is AV 11 20 68. I am replacing that rear with one with a 3.70 ratio, the code on this one is AV 11 25 68. The problem I have is that the driveshaft u-joints on the car when I took it apart are 3 5/8" wide front and rear. The u-joints that I bought for the change are 3 1/4" wide (the catalogue only showed one u-joint for this year car), also the 3.70 rear driveshaft yoke is made for the 3 1/4" u-joints. on other difference is that on the 3.08 rear the u-joint was clamped in place with a strap, and two bolts on each side, while on the 3.70 rear a u-bolt was used on each side. Can anyone explain why these two rears are different? Is there a difference between 4 spd, & automatic rears? What if anything can I do to make this swap work without buying a new driveshaft and yoke? thanks for any info.

[Joe L.]

Re: swapping rear in a 69

Greg-----

For 1968-70, Corvettes equipped with automatic transmissions used a 3-5/8" u-joint yoke diameter. Cars with manual transmissions used the 3-1/4" yoke diameter. The 3.70:1 differential unit that you have was originally installed in a 69 Corvette with manual transmission (this makes sense, too, since the 3.70:1 axle ratio was never originally used for auto trans applications in 1969).

The best way to convert the 3.70:1 rear that you have for use with your existing drivetrain is to replace the companion flange on the differential unit with the 3-5/8" unit from your existing differential. They are completely interchangeable. However, you do need to be careful regarding proper torqing of the pinion shaft nut when you make the swap.

Alternatively, you can obtain a conversion u-joint in the aftermarket. The one that you need will have the 3-1/4" finger diameter for one opposing pair of trunnions and the 3-5/8" finger diameter for the other opposing pair of trunnions.

[Greg 32781]

thanks Joe *NM*