Sandy,
Course thread is what you want in Aluminum due to the lack of strength of aluminum versus steel. Not to say that Aluminum is an inferior material, just stating facts. Normally the more threads you have (finer threads) the more holding power you have, but this is in steel, so Bill was on the right thought pattern, but in your case your talking about Aluminum. Course threads are the way to go. We (Eaton Corp) use course threads in all of our Super Charger cases for the reasons above.
Aluminum Tap Magic is great for tapping or machining aluminum.
MicoCut works well too for tapping Aluminum and steel.
Good luck,