Quote:
Originally Posted by David.X5
You will get nicer threads if you start with the correct size drilled hole (nice and straight, not wobbly). And less likely to break the tap. The correct tap drill for a m20x1.5 in Aluminum is 18.5 mm (close to 47/64 inch).
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Totally agree with ya, David.X5 - but with a stripped out 18mm threaded hole, there isn't going to be much to drill out!
As long as the thread pitch is the same (and there are - 1.5mm) then the new tap should run up the remains of the old thread pretty cleanly. It might bite a bit too much, but use plenty of grease in the tap's flutes, both to catch any swarf/chips and to lubricate the cutting faces on the tap. Go slowly, in about 1/6th of a turn, then back out to clear the swarf.