Battery ground to aluminum head?

Aluminum is an excellent conductor of electricity.
With exposure to air and over time aluminum surfaces will oxidize and form a layer of aluminum oxide.
Aluminum oxide is a poor conductor of electricity.
Some aluminum alloys have their surfaces clad with an aluminum oxide layer to inhibit corrosion.
You can test the surface of the aluminum for conductivity by using a continuity tester (light) or a meter.

Therefore, you should prepare an aluminum surface for grounding or conducting electricity by removing any oxide layer that may have built up. You can use Scotchbrite pads, sandpaper, Emory cloth, wire wheel, file, etc to remove the oxide layer and produce a fresh shiny or bright surface.