When tapping into aluminum, bar soap works quite well as a cutting aid, just coat the tap with soap. The soap hold the chips so that you can easily remove the tap every turn or so and clean off the chips. Aluminum (when tapping very small sizes like #2 or #4 seems to be more critical for binding during the tapping process than iron or steel does. As an aside, I definitely do NOT recommend that you use WD40 as a cutting oil. WD40 is not a lubricant, it is a water dispersant fluid and a very poor lubricant. Plain old motor oil is a much better fluid if you do not have cutting fluid.