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As a guide, a tapping drill is the diameter less the pitch plus a bit to the nearest you have, that is for 60 degree threads, some are 55 degree near enough though. For example M3 x 0.5mm, a 2.5mm drill is perfect, M8 x 1.25, 6.75mm, who has one of those, go for 6.8mm.
Believe it or not spit is a great lubricant for tapping aluminium. Any old oil will work but once you try a proper tapping compound you will never look back.