Run to Rund

The Olds Type O differential uses 7/16" bolts which are stronger than the 12 bolt Type C that uses 3/8" bolts to hold the ring gear.

As to whether the car should remain a 6, get an Olds V8, or some modern "off-brand" V8, it is largely personal preference. It is easy to get into arguments with factory vs. owner or dealer installed options, and what makes the car "not real" or a "clone." If adding some options correctly is OK, then others should be OK too; the 442 option was just that in 1964-67, just like a radio, wheel covers, or power steering. Correct conversion is the key to a reasonable end product.

The owner of this car just needs to understand that there are various levels of swaps or builds, of varying complexities. First set the "theme" for the car, and then the hundreds of decisions will fall into place. At the finish, if the car does not satisfy your expectations, you will end up selling it and starting over again. If you don't want a low horsepower 6 cylinder, don't build the car that way. If it has to be a "real" meaning factory built 442, then get one with the correct VIN and cowl tag, rather than adding the parts to your 3307 car. If it is more important to have a rust-free shell and to have fun with higher horsepower, go ahead with the V8 swap, etc.

joe_padavano

Here's where math makes life interesting. I ***-U-ME-d that the larger bolt circle diameter of the 12 bolt would make up for the smaller bolt diameter. Silly me:

The root area of a 3/8" fine thread bolt is 0.082 sq in, 7/16" is 0.110 sq in. That's a 35% increase in shear area and thus a 35% increase in torque capacity for a ring gear with the SAME BOLT CIRCLE and SAME NUMBER OF BOLTS.

Interestingly, using 12 of the 3/8" bolts vs. 10 of the 7/16" bolts on the same bolt circle, the ten 7/16" bolts STILL have a greater ring gear torque capacity, but the advantage is only 12%.

Increasing the bolt circle diameter of the 12 bolt results in the Type O STILL having greater ring gear torque capacity, but further reduced to only an 8% advantage. Note that not having the exact bolt circles, I simply increased the torque capacity by the ratio of the ring gear diameters (8.875/8.5).

Now, bolt shear is not the only factor that affects rear axle strength, but it is an important one. The larger ring gear will result in lower stresses on individual ring gear teeth, for example. I must say that based on this, one cannot say that the Chevy 12 bolt is obviously stronger. The other thing this exercise demonstrates is the power of the r-squared factor in the area calculation.