[Espeefan]

I buy my taps from McMaster-Carr too. Sometimes you are limited on the coatings they have available, depending on the size, but one thing I have noticed is that the black oxide or oxide over nitride coatings really help in aluminum. Those coated taps doesn't plug up nearly as easily as titainum nitride coated taps. There is nothing worse then having the tip of a spiral point tap plug with aluminum on you, while you are tapping. It cuts good threads on the way down, but as you back it up, it tears the threads out, which you just tapped. Even if you are especially careful with backing the tap out every one turn in, and use plenty of good oil, it can still happen. Once the tap is plugged, it's junk. Aluminum tends to weld itself to the tap.