If you’re concerned about chips, you could also try a back-tap like this one.
http://www.shopcompanion.com/KD+Tools+3689+Back+Tap+12+Millimeter+Reverse+Spark+Plug+Re+Thread+Tool+-303835328-o+.xhtml
A Back-tap is inserted into the hole, the center shaft tightened to pull the expander back and expand the cutting parts to full diameter, and then the tapping is done during extraction rather than insertion. As with all metal cutting tools, lubrication should be used, in this case a bit of grease.