[BrianG]

In my first post, it's easy to see the teeth have different geometries; not only is the top and base of the tooth different size, the general shape is different too.

I wonder if the pinions in Mike's store are simply hardened versions of regular pinions? If any one has a clear close-up of one of Mike's hardened pinions, could you post that picture? Please?

I know that using the Delrin/plastic spur with a regular pinion is cheaper than using a few hardened gears. However, if a diff "goes" and you just want to replace it to keep racing, you can simply get the whole diff at more places and install it in a matter of minutes. Otherwise, you'd have to have the diff ahead of time pre-installed with the plastic spur.

I would think (please correct me if I'm wrong here) that a steel setup would be a tad more tolerant of a slight momentary mis-mesh. If you take a large jump and land hard, the weight of the motor coupled with inertia may flex the aluminum a little and the mesh might not be perfect. At that point, less of the teeth are in contact. The very tops of a few teeth of a plastic spur would probably have some shearing.