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@jmorris Here's one reference. Can't remember where I first learned the trick though...

https://bbs.homeshopmachinist.net/forum/general/1927244-broken-tap-removal-with-alum

It's potentially an all-day process.

Also, besides thread forming taps, have you tried spiral flute taps? I've had good luck with them, particularly ones with polished flutes expressly made for aluminum. Mine are from YG-1.